2.6 User-Defined Functions

User-defined functions provide a way to group together related statements into a cohesive block. For reusable code, a function saves duplicating statements and makes maintenance of the code easier. Consider an example of a simple user-developed function as shown in Example 2-3.

Example 2-3. A user-defined function to output bold text

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"

                      "http://www.w3.org/TR/html401/loose.dtd">

<html>

<head>

  <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">

  <title>Simple Function Call</title>

</head>

<body bgcolor="#ffffff">

<?php



function bold($string)

{

    print "<b>" . $string . "</b>";

}



// First example function call (with a static string)

print "this is not bold ";

bold("this is bold ");

print "this is again not bold ";



// Second example function call (with a variable)

$myString = "this is bold";

bold($myString);

?>

</body></html>

The script defines the function bold( ), which takes one parameter, $string, and prints that string prefixed by a bold <b> tag and suffixed with a </b> tag. The parameter $string is a variable that is available in the body of the function, and the value of $string is set when the function is called. As shown in the example, the function can be called with a string literal expression or a variable as the parameter.

Functions can also return values. For example, consider the following code fragment that declares and uses a function heading( ), which returns a string using the return statement:

function heading($text, $headingLevel)

{

    switch ($headingLevel)

    {

    case 1:

        $result = "<h1>$text</h1>";

        break;



    case 2:

        $result = "<h2>$text</h2>";

        break;



    case 3:

        $result = "<h3>$text</h3>";

        break;



    default:

        $result = "<p><b>$text</b></p>";

    }

    return($result);

}



$test = "User-defined Functions";

print heading($test, 2);

The function takes two parameters: the text of a heading and a heading level. Based on the value of $headingLevel, the function builds the HTML suitable to display the heading. The example outputs the string:

<h2>User-defined Functions</h2>

The variable that is returned by a return statement can optionally be placed in parentheses: the statements return($result) and return $result are identical.

2.6.1 Parameter Types and Return Types

The parameter and return types of a function aren't declared when the function is defined. PHP allows parameters of any type to be passed to the function, and as with variables, the return type is determined when a result is actually returned. Consider a simple function that divides two numbers:

function divide($a, $b)

{

    return ($a/$b);

}

The value returned from the function divide( ) is the value of the expression ($a/$b). The type that is returned depends on the parameters passed to divide( ). For example:

$c = divide(4, 2);     // assigns an integer value = 2

$c = divide(3, 2);     // assigns a float value = 1.5

$c = divide(4.0, 2.0); // assigns a float value = 2.0

If the types of parameters passed to the function are critical, they should be tested as shown earlier in Section 2.5.1.

The return statement causes the execution of the function to end. To illustrate this, consider an improved divide( ) function definition that tests the parameter $b to avoid divide-by-zero errors:

function divide($a, $b)

{

    if ($b == 0)

        return false;



    return ($a/$b);

}

If $b is 0, then the function returns false and the division of $a/$b is never executed.

The return statement can also be used to exit from functions that don't return values. Consider the following definition of bold( ) that simply prints the parameter $string without any bold mark-up when passed non-string values:

function bold($string)

{

    if (! is_string($string))

    {

        print $string;

        return;

    }

    print "<b>" . $string . "</b>";

}

2.6.2 Variable Scope

Variables used inside a function are different from those used outside a function. The variables used inside the function are limited to use within the function. This is called the scope of the variable. There are exceptions to this rule, which are discussed later in this section. Consider an example that illustrates variable scope:

function doublevalue($var)

{

    $temp = $var * 2;

}



$variable = 5;

doublevalue($variable);

print "\$temp is: $temp";

This example outputs the string:

$temp is:

with no value for $temp. The scope of the variable $temp is local to the function doublevalue( ) and is discarded when the function returns.

The PHP script engine doesn't complain about an undeclared variable being used. It just assumes the variable is empty. However, this use of an undefined variable can be detected by configuring the error-reporting settings. Error reporting is discussed in Chapter 14.

The easiest way to use a value that is local to a function elsewhere in a script is to return the value from the function with the return statement. The calling script can simply assign the returned value to a local variable. The following example does this:

function doublevalue($var)

{

    $returnVar = $var * 2;

    return($returnVar);

}



$variable = 5;

$temp = doublevalue($variable);

print "\$temp is: $temp";

The example prints:

$temp is: 10

You could have still used the variable name $temp inside the function doublevalue( ). However, the $temp inside the function is a different variable from the $temp outside the function. The general rule is that variables used exclusively within functions are local to the function, regardless of whether an identically named variable is used elsewhere. There are three exceptions to this general rule: variables passed by reference, variables declared global in the function, and superglobals that contain user and environment values and are automatically created by PHP at runtime. Global variables are discussed in the next section, and superglobals are discussed in Chapter 6.

2.6.2.1 Global variables

If you want to use the same variable everywhere in your code, including within functions, you can do so with the global statement. The global statement declares a variable within a function as being the same as the variable that is used outside of the function. Consider this example:

function doublevalue( )

{

    global $temp;

    $temp = $temp * 2;

}



$temp = 5;

doublevalue( );

print "\$temp is: $temp";

Because $temp is declared inside the function as global, the variable $temp used in doublevalue( ) is a global variable that can be accessed outside the function. Because the variable $temp can be seen outside the function, the script prints:

$temp is: 10

A word of caution: avoid overuse of global as it makes for confusing code.

The global variable declaration can be a trap for experienced programmers. In some other languages, global variables are usually declared global outside the functions and then used in the functions.

In PHP, it's the opposite: to use a global variable inside a function, declare the variable as global inside the function.

Allowing a function to modify global variables solves the problem that a return statement can only pass back one value. An alternative to using global is to return an array of values—this approach becomes clear when we discuss arrays in Chapter 3. A better approach is to pass parameters by reference instead of by value, a practice described later.

2.6.2.2 Static variables

Variables can also be declared within a function as static. The static variable is available only in the scope of the function, but the value is not lost between function calls. Consider simple function count( ) that declares a static counter variable $count:

function count( )

{

    static $count = 0;

    $count++;

    return $count;

}



// prints 1

print count( );



// prints 2

print count( );

The first time the function count( ) is called, the static variable $count is set to zero, and incremented. The value of $count is maintained for subsequent calls.

2.6.3 Passing Variables to Functions

By default, variables are passed to functions by value, not by reference. Consider an example:

function doublevalue($var)

{

    $var = $var * 2;

}      



$variable = 5;

doublevalue($variable);

print "\$variable is: $variable";

This produces the output:

$variable is: 5

The parameter $variable that is passed to the function doublevalue( ) isn't changed by the function. What actually happens is that the value 5 is passed to the function, doubled to be 10, and the result lost forever! The value is passed to the function, not the variable itself.

2.6.3.1 Passing parameters by reference

An alternative to returning a result or using a global variable is to pass a reference to a variable as a parameter to the function. This means that any changes to the variable within the function affect the original variable. Consider this example:

function doublevalue(&$var)

{

    $var = $var * 2;

}



$variable = 5;

doublevalue($variable);

print "\$variable is: $variable";

This prints:

$variable is: 10

The only difference between this example and the previous one is that the parameter $var to the function doublevalue( ) is prefixed with an ampersand character: &$var. The effect is a bit to hard to understand unless one learns low-level computer languages, but it means that the parameter doesn't contain the value of the variable—instead, it points to where the variable is stored in memory. The result is that changes to $var in the function affect the original variable $variable outside the function.

If a parameter is defined as a reference, you can't pass the function a literal expression as that parameter because the function expects to modify a variable. PHP reports an error when the following is executed:

function doublevalue(&$var)

{

    $var = $var * 2;

}



// The following line causes an error

doublevalue(5);

2.6.3.2 Assigning by reference

Referencing with the ampersand can also be used when assigning variables, which allows the memory holding a value to be accessed from more than one variable. This example illustrates the idea:

$x = 10;

$y = &$x;

$y++;

print $x;

print $y;

This fragment prints:

11

11

Because $y is a reference to $x, any change to $y affects $x. In effect, they are the same variable. The reference $y can be removed with:

unset($y);

This has no effect on $x or its value.

Assigning variables with a reference to another variable can also be done with the reference assignment operator =& with exactly the same outcome as shown in the previous example. The following fragment sets up three variables—$x, $y, and $z—that all point to the same value:

$x = 10;



// Use the reference assignment operator =& to assign a reference to $x

$y =& $x;



// Use the assignment operator = to copy a reference to $x

$z = &$x;



$x = 100;



// Prints "x = 100,  y = 100,  z = 100"

print "x = {$x},  y = {$y},  z = {$z}";

2.6.3.3 Default parameter values

PHP allows functions to be defined with default values for parameters. A default value is simply supplied in the parameter list using the = sign. Consider the heading( ) function described earlier; here we modify the function definition to include a default value:

function heading($text, $headingLevel = 2)

{

    switch ($headingLevel)

    {

    case 1:

        $result = "<h1>$text</h1>";

        break;



    case 2:

        $result = "<h2>$text</h2>";

        break;



    case 3:

        $result = "<h3>$text</h3>";

        break;

    default:

        $result = "<p><b>$text</b></p>";

    }



    return($result);

}



$test = "User-defined Functions";

print heading($test);

When calls are made to the heading( ) function, the second argument can be omitted, and the default value 2 is assigned to the $headingLevel variable.

2.6.4 Reusing Functions with Include and Require Files

It's valuable to be able to reuse functions in many scripts. PHP provides the include and require statements that allow you to reuse PHP scripts containing statements, function definitions, and even static HTML.

If you decide to reuse the bold( ) function from Example 2-3 in more than one script, you can store it in a separate include file . For example, you can create a file called functions.inc and put the bold( ) function in the file:

<?php

function bold($string)

{

    print "<b>" . $string . "</b>";

}

?>

Any PHP code in an include file must be surrounded by the PHP start and end script tags. The PHP script engine treats the contents of include files as HTML unless script tags are used.

You can then use include to provide access to the bold( ) function:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"

                      "http://www.w3.org/TR/html401/loose.dtd">

<html>

<head>

  <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">

  <title>Simple Function Call</title>

</head>

<body bgcolor="#ffffff">

<?php

include "functions.inc";



// First example function call (with a string expression)

print "this is not bold ";

bold("this is bold ");

print "this is again not bold ";



// Second example function call (with a variable)

$myString = "this is bold";

bold($myString);

?>

</body></html>

Include files can also be used to incorporate resources such as static HTML or a set of variable initializations. The following example could be written to the file release.inc and included in all the scripts of an application:

<!-- Beta Release Only  -->

<?php

    $showDebug = true;

?>

Both include and require read external include files, the only difference is in the behavior when a file can't be included: include provides a warning whereas require terminates the script with a fatal error.

When you are including a file that contains user-defined functions, or other mandatory content, you should use the require directive. We use the require directive in all of our code.

The include and require statements can be treated in the same way as other statements. For example, you can conditionally include different files using the following code fragment:

if ($netscape == true)

{

    require "netscape.inc";

}

else

{

    require "other.inc";

}

The file is included only if the include statement is executed in the script. The braces used in this example are necessary: if they are omitted, the example doesn't behave as expected.

Scripts can include more than one include file, and include files can themselves include other files. Writing scripts that use include or require can lead to an include file being included and evaluated twice. To avoid problems with variable reassignments and function redefinitions, PHP provides the include_once or require_once constructs statements that ensure that the contents of the file are included only once.

2.6.4.1 Managing include files

As you develop reusable code, you should consider how you will arrange your include files. By default, when a file is included using the include or require statements, PHP searches for the file in the same directory as the script being executed. You can include files in other directories by specifying a file path in the include or require statements. The following example shows how relative and absolute file paths can be used:

// a relative file path

require "../inc/myFunctions.php";



// an absolute file path

require "/library/database/db.inc";

The paths can be specified with forward slashes for both Unix and Microsoft Windows environments, allowing scripts to be moved from one environment to another. However, using paths can make it difficult to change the directory structure of your application.

A more sophisticated, and flexible alternative to accessing include files is to set the include_path parameter defined in the php.ini configuration file. One or more directories can be specified in the include_path parameter, and when set, PHP will search for include files relative to those directories. The following extract from the php.ini file shows how to set the include_path parameter:

;;;;;;;;;;;;;;;;;;;;;;;;;

; Paths and Directories ;

;;;;;;;;;;;;;;;;;;;;;;;;;



; UNIX: "/path1:/path2"  

;include_path = ".:/php/includes:/usr/local/php/projectx"

;

; Windows: "\path1;\path2"

include_path = ".;c:\php\includes;d:\php\projectx"

Path specifications for this parameter are system specific. Unix paths use the forward slash and are separated with the colon (:) character, while Microsoft Windows paths use the backslash and are separated by semi colons (;).

The php.ini configuration file defines many parameters that are used to define aspects of PHP's behavior. Whenever you change php.ini, you need to restart your Apache web server so that the changes are re-read; instructions for restarting are in Appendix A.

If you set the include_path parameter, include and require directives need only specify a path relative to a directory listed in the include_path. For example, if the include_path is set to point at /usr/local/php/projectx, and you have an include file security.inc that's stored in /usr/local/php/projectx/security, you only need to add:

include "security/security.inc";

to your script file. The PHP engine will check the directory /usr/local/php/projectx, and locate the subdirectory security and its include file. Include files that are placed in directories outside of the web server's document root are protected from accessed via the web server. In Chapter 6 we describe how to protect include files that are under the web server root directory.

For a large project, you might place the project-specific code into one directory, while keeping reusable code in another; this is the approach we use in our case study, Hugh and Dave's Online Wines, as we describe in Chapter 15.

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