PHP/Utility Function/include once

include_once and require_once will only include a file once

 
//bar.php:
    <?php
            print "In bar\n";
    ?>
//index.php
    <?php
            include_once "bar.php";
            include_once "BAR.php";
            include_once "Bar.php";
    ?>

include_once() function verifies whether or not the file has already been included.

 
If it has been included, include_once() will not execute. 
Otherwise, it will include the file as necessary. 
Its syntax follows: include_once (file insertion_file)

Using include_once to include a file

 
A sample include file called add.php
<?php
function add( $x, $y ){
  return $x + $y;
}
?>

<?php
include_once("add.php");
include_once("add.php");
echo add(2, 2);
?>