The require() statement replaces itself with the specified file, much like the C preprocessor's #include works.
If "URL fopen wrappers" are enabled in PHP (which they are in the default configuration), you can specify the file to be require()ed using an URL instead of a local pathname. See Remote files and fopen() for more information.
An important note about how this works is that when a file is include()ed or require()ed, parsing drops out of PHP mode and into HTML mode at the beginning of the target file, and resumes PHP mode again at the end. For this reason, any code inside the target file which should be executed as PHP code must be enclosed within valid PHP start and end tags.
require() is not actually a function in PHP; rather, it is a language construct. It is subject to some different rules than functions are. For instance, require() is not subject to any containing control structures. For another, it does not return any value; attempting to read a return value from a require() call results in a parse error.
Unlike include(), require() will always read in the target file, even if the line it's on never executes. If you want to conditionally include a file, use include(). The conditional statement won't affect the require(). However, if the line on which the require() occurs is not executed, neither will any of the code in the target file be executed.
Similarly, looping structures do not affect the behaviour of require(). Although the code contained in the target file is still subject to the loop, the require() itself happens only once.
This means that you can't put a require() statement inside of a loop structure and expect it to include the contents of a different file on each iteration. To do that, use an include() statement.
require ('header.inc');
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When a file is require()ed, the code it contains inherits the variable scope of the line on which the require() occurs. Any variables available at that line in the calling file will be available within the called file. If the require() occurs inside a function within the calling file, then all of the code contained in the called file will behave as though it had been defined inside that function.
If the require()ed file is called via HTTP using the fopen wrappers, and if the target server interprets the target file as PHP code, variables may be passed to the require()ed file using an URL request string as used with HTTP GET. This is not strictly speaking the same thing as require()ing the file and having it inherit the parent file's variable scope; the script is actually being run on the remote server and the result is then being included into the local script.
/* This example assumes that someserver is configured to parse .php
* files and not .txt files. Also, 'works' here means that the variables
* $varone and $vartwo are available within the require()ed file. */
/* Won't work; file.txt wasn't handled by someserver. */
require ("http://someserver/file.txt?varone=1&vartwo=2");
/* Won't work; looks for a file named 'file.php?varone=1&vartwo=2'
* on the local filesystem. */
require ("file.php?varone=1&vartwo=2");
/* Works. */
require ("http://someserver/file.php?varone=1&vartwo=2");
$varone = 1;
$vartwo = 2;
require ("file.txt"); /* Works. */
require ("file.php"); /* Works. */
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In PHP3, it is possible to execute a return statement inside a require()ed file, as long as that statement occurs in the global scope of the require()ed file. It may not occur within any block (meaning inside braces ({}). In PHP4, however, this ability has been discontinued. If you need this functionality, see include().
See also include(), require_once(), include_once(), readfile(), and virtual().