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include require

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require

require is similar to include, except that it will produce a fatal E_COMPILE_ERROR level error on failure. When the require fails, it will halt the script. When the include fails, it will not halt the script and only emit E_WARNING. 

require 'file.php';

PHP Manual - Control Structures - Require

include

The include statement includes and evaluates a file.

./variables.php 
$a = 'Hello World!';
./main.php` 
include 'variables.php';
echo $a;
// Output: `Hello World!`

Be careful with this approach, since it is considered a code smell, because the included file is altering amount and content of the defined variables in the given scope.

You can also include file, which returns a value. This is extremely useful for handling configuration arrays:

configuration.php 
<?php 
return [
    'dbname' => 'my db',
    'user' => 'admin',
    'pass' => 'password',
];
main.php 
<?php
$config = include 'configuration.php';

This approach will prevent the included file from polluting your current scope with changed or added variables.

PHP Manual - Control Structures - Include

include & require can also be used to assign values to a variable when returned something by file.

Example :

include1.php file : 

<?php
    $a = "This is to be returned";

    return $a;
?>

index.php file : 

$value = include 'include1.php';
// Here, $value = "This is to be returned"
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Control structures:
* Control Structures
* if else
* Alternative syntax for control structures
* do-while
* goto
* declare
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* include require
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* if elseif else
* if
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