PHP include() Vs require()

by Vincy. Last modified on July 1st, 2022.

In this tutorial, we are going to see the list of functions used in PHP to include an external file into a program. PHP provides various functions including external files.

It is required if the current code dependency is on the external file to be included. For example, if we want to create an instance of a class defined in a separate class file, then it has to be included before creating an instance of it.

The following list of functions is used to include an external file in a PHP program. In this tutorial, we are going to compare these functions with suitable examples.

Also, we are going to see the purpose of the _once usage and the difference between include and include_once/require and require_once.

  1. include()
  2. require()
  3. include_once()
  4. require_once()


PHP include() function includes external file into a PHP program. It accepts the external file path and checks if the file exists or not. If the file does not exist in the specified path, then the include() will return a PHP warning.

Warning: failed to open stream: No such file or directory...
Warning: Failed opening ... for inclusion...

By including an external file by using the PHP include() function, the variable, functions, and classes of the included file can be used in the program where it is included. The following code shows an example for including an external file using PHP include().

include ("../file_name.php"); // relative path
                              // OR
include ("/xampp/hddocs/file_name.php"); // absolute path

If we include the same file multiple times by using this function, then it will cause a PHP error.


PHP require()  function is as similar as include() function. But, the difference is, that the ‘require()’ function will return a fatal error and stop executing the program at the time of failure whereas the include() function returns a warning and continue execution.

Warning: failed to open stream: No such file or directory...
Fatal error: Failed opening required...

The code to include file using require() function is,

require ("../file_name.php"); // relative path
                              // OR
require ("/xampp/hddocs/file_name.php"); // absolute path

include_once() and require_once():

The include_once() and require_once() functions are similar to the include() and require() functions, respectively. But, using this function will create a difference in the time of including the same file multiple times.

By using include_once() and require_once() functions, will include the specified file if it is not already included, otherwise, PHP will ignore this statement. The code is,

include_once ("../file_name.php"); // relative path
require_once ("../file_name.php"); // relative path
Written by Vincy, a web developer with 15+ years of experience and a Masters degree in Computer Science. She specializes in building modern, lightweight websites using PHP, JavaScript, React, and related technologies. Phppot helps you in mastering web development through over a decade of publishing quality tutorials.

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