I need to mock a php native function

I had the same problem a few months ago. This is what worked for me:

You could namespace your test with the namespace of the class under test. Then, before the test class, create the function in that namespace, like this:

 namespace The\Same\Like\The\Class\Under\Test

use \Mockery;

function shell_exec($cmd)
{
  return YourTest::$functions->shell_exec($cmd);
}

class YourTest extends \PHPUnit_Framework_TestCase {
  public static $functions;

  public function setUp()
  {
    self::$functions = Mockery::mock();
  }
}

Now, when you're testing your class, and want to mock the function, you could write it like this:

    self::$functions->shouldReceive('shell_exec')->with($cmd)->once();

Or in whatever way you want to mock it.

In phpspec:

This is exactly why I didn't use phpspec when I had this problem.

I think (but not sure) that with a phpspec approach in mind, you would wrap the shell command in a class, like this:

class Shell{
  public function exec($cmd)
  {
    return shell_exec($cmd);
  }
}

I guess, there's no need to test this class, since it's just a wrapper. Then, you would use this class instead of the function in your other classes, and when it comes to mocking, you could mock this class.

reference : https://laracasts.com/discuss/channels/testing/i-need-to-mock-a-php-function