PHP Manual
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require_once()

The require_once() statement replaces itself with the specified file, much like the C preprocessor's #include works, and in that respect is similar to the require() statement. The main difference is that in an inclusion chain, the use of require_once() will assure that the code is added to your script only once, and avoid clashes with variable values or function names that can happen.

For example, if you create the following 2 include files utils.php and foolib.php
Example 11-3. utils.php
<?php define("PHPVERSION", floor(phpversion())); echo "GLOBALS ARE NICE\n"; function goodTea() { return "Oolong tea tastes good!"; } ?>

Example 11-4. foolib.php
<?php require ("utils.php"); function showVar($var) { if (PHPVERSION == 4) { print_r($var); } else { var_dump($var); } } // bunch of other functions ... ?>
And then you write a script cause_error_require.php
Example 11-5. cause_error_require.php
<?php require("foolib.php"); /* the following will generate an error */ require("utils.php"); $foo = array("1",array("complex","quaternion")); echo "this is requiring utils.php again which is also\n"; echo "required in foolib.php\n"; echo "Running goodTea: ".goodTea()."\n"; echo "Printing foo: \n"; showVar($foo); ?>
When you try running the latter one, the resulting ouptut will be (using PHP 4.01pl2):

GLOBALS ARE NICE
GLOBALS ARE NICE

Fatal error:  Cannot redeclare goodTea() in utils.php on line 5

By modifying foolib.php and cause_errror_require.php to use require_once() instead of require() and renaming the last one to avoid_error_require_once.php, we have:

Example 11-6. foolib.php (fixed)

...
require_once("utils.php");
function showVar($var)
{
...

Example 11-7. avoid_error_require_once.php

...
require_once("foolib.php");
require_once("utils.php");
$foo = array("1",array("complex","quaternion"));
...

And when running the latter, the output will be (using PHP 4.0.1pl2):

GLOBALS ARE NICE
this is requiring globals.php again which is also
required in foolib.php
Running goodTea: Oolong tea tastes good!
Printing foo:
Array
(
    [0] => 1
    [1] => Array
        (
            [0] => complex
            [1] => quaternion
        )

)

Also note that, analogous to the behavior of the #include of the C preprocessor, this statement acts at "compile time", e.g. when the script is parsed and before it is executed, and should not be used for parts of the script that need to be inserted dynamically during its execution. You should use include_once() or include() for that purpose.

For more examples on using require_once() and include_once(), look at the PEAR code included in the latest PHP source code distributions.