Will require_once and include_once together include the file twice?

If I have this:

require_once('myfile.php');

then later:

include_once('myfile.php');

Will the file get included again or just the once?

Also what is the difference between the two? Require causes an error on failure and include tries to recover? any other differences?

If the file is included, it is included.

requice_once() works exactly like include_once(), except that it kills the script when the script to include is not found.

Therefore in your example, the script is included once.

Conceptually: Include does what it says on the tin: it includes the code of 'myfile.php' in the code of the current file. Require is more like an import statement in other programming languages.

The _once suffix means that PHP avoids inlcuding/requiring the file if it is already available to the current script via a previous include/require statement or recursively via include/require statements in other imported scripts.

Will the file get included again or just the once?

myfile.php is just included the once.

require_once 'myfile.php'; //included
include_once 'myfile.php'; // not included

What is the difference between the two?

• include and include_once will both emit a warning if it cannot find a file.

• require and require will emit a fatal error if it cannot find a file.

The difference between include_once and require is that when they cannot find the file include_once will emit a warning and require will emit a fatal error.

require is identical to include except upon failure it will also produce a fatal E_COMPILE_ERROR level error. In other words, it will halt the script whereas include only emits a warning (E_WARNING) which allows the script to continue. [http://php.net/manual/en/function.require.php]

Require causes an error on failure and include tries to recover? any other differences?

include allows you to recover yes, a failed require statement is always ending one way, a try catch cant even save your script.

Its worth pointing out what would happen if other combinations of the four file evaluation statments (include, include_once ,require and require) were used.

If the file were found

include and require will always include the file regardless of whether you have previously used require,include, requireor include_once with the same file.

include_once 'myfile.php'; //included
include 'myfile.php'; //always included
require 'myfile.php'; //always included
require_once 'myfile.php'; // not included, prevously included via [function.include_once], [function.include] and [function.require]

include_once and require will only include the file if it has not been previously included using a require,include, require or include_once statement.

require 'myfile.php'; //always included
include_once 'myfile.php'; // not included, prevously included via [function.require]