PHP mocking built-in functions (e,g exec, file_exists etc.) for testing

SomeClassTest.php

<?php
/**
 * Example of mocking built-in functions for testing
 *
 * @Author    : ajaxray <anis.programmer@gmail.com>
 * @Date-Time : 03/05/2017
 * 
 * How it works?
 * ----------------
 * Simply by redefining the function for target Namespace.
 * Note : we can mock if global functions are used as "file_exist()", but not as "\file_exist()"
 */
namespace Namespace\Of\Test\Class;

use Namespace\Of\Class\Under\Test;

class SomeClassTest extends \PHPUnit_Framework_TestCase
{
    protected function setUp()
    {
        global $mockGlobalFunctions;
      
        $mockGlobalFunctions = true;
        // setting flase to $mockGlobalFunctions will de-activate mocking
    }

    public function testSomethingThatRunsExec()
    {
        global $lastExecCommand;
      
        // Perform the test
        $this->assertEquals('expected command -and -args', $lastExecCommand);
    }
}

// ===================================================================================
// Mechanism for mocking some built in functions ()
// This section is placed at the bottom of SomeClassTest file
// ==================================================================================
namespace Namespace\Of\Class\Under\Test;

$GLOBALS['mockGlobalFunctions'] = false;
$GLOBALS['lastExecCommand'] = null;

// Overwriting `file_exists` to always return true
function file_exists($path)
{
    global $mockGlobalFunctions;

    if (isset($mockGlobalFunctions) && $mockGlobalFunctions === true) {
        return true;
    } else {
        return call_user_func_array('\file_exists', func_get_args());
    }
}

// Overwriting `mime_content_type` to return a planned mime type
function mime_content_type($path)
{
    global $mockGlobalFunctions;

    if (isset($mockGlobalFunctions) && $mockGlobalFunctions === true) {
        if(preg_match('/(png)|(jpe?g)|(gif)/', $path, $match)) {
            return 'image/'. $match[0];
        } elseif (preg_match('/(pdf)|(x\-pdf)/', $path, $match)) {
            return 'application/'. $match[0];
        }
        return 'no-pdf/no-image';
    } else {
        return call_user_func_array('\mime_content_type', func_get_args());
    }
}

// Overwriting `exec` to set the command in a global variable instead of executing
function exec($command, $output, $returnCode)
{
    global $mockGlobalFunctions, $lastExecCommand;

    if (isset($mockGlobalFunctions) && $mockGlobalFunctions === true) {
        $lastExecCommand = func_get_arg(0);
    } else {
        return call_user_func_array('\exec', func_get_args());
    }
}