Check if a file was included or loaded

you can do this by get_included_files — Returns an array with the names of included or required files and validate against __FILE__

I appreciate all the answers, but I didn't want to use any one's solution here, so I combined your ideas and got this:

<?php
    // place this at the top of the file
    if (count(get_included_files()) == 1) define ('TEST_SUITE', __FILE__);

    // now I can even include bootstrap which will include other
    // files with similar setups
    require_once '../bootstrap.php'

    // code ...
    class Bar {
        ...
    }
    // code ...

    if (defined('TEST_SUITE') && TEST_SUITE == __FILE__) {
        // run test suite here  
    }
?>

Quoted from: How to know if php script is called via require_once()?

I was looking for a way to determine if a file have been included or called directly, all from within the file. At some point in my quest I passed through this thread. Checking various other threads on this and other sites and pages from the PHP manual I got enlightened and came up with this piece of code:

if (basename(__FILE__) == basename($_SERVER["SCRIPT_FILENAME"])) {
  echo "called directly";
} else {
  echo "included/required";
}

In essence it compares if the name of the current file (the one that could be included) is the same as the file that is beeing executed.

Credit: @Interwebs Cowboy

if (defined('FLAG_FROM_A_PARENT'))
// Works in all scenarios but I personally dislike this

if (__FILE__ == get_included_files()[0])
// Doesn't work with PHP prepend unless calling [1] instead.

if (__FILE__ == $_SERVER['SCRIPT_FILENAME'])
// May break on Windows due to mixed DIRECTORY_SEPARATOR

if (basename(__FILE__) == basename($_SERVER['SCRIPT_FILENAME']))
// Doesn't work with files with the same basename but different paths

if (realpath(__FILE__) == realpath($_SERVER['SCRIPT_FILENAME']))
// Seems to do the trick as long as document root is properly configured

Note: On WAMP Servers virtual-hosts sometimes inherit the default document root setting, causing $_SERVER['DOCUMENT_ROOT'] to display wrong path.