Is it better to use require_once('filename.php') or require_once 'filename.php';

Pear Coding Standards say :

"include_once and require_once are statements, not functions. Parentheses should not surround the subject filename."

Source : http://pear.php.net/manual/en/standards.including.php

It's exactly the same thing. It's a matter of style.

The parentheses may get in the way some times. For instance, this example from the manual doesn't do what you expect:

if (include('vars.php') == 'OK') {
    echo 'OK';
}

See example #4.

What does your heart tell you?

Performance difference, if any: negligible.

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