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5 years ago
Important note: When the imagedestroy is called, the resource is freed but the printed resource output is the same as before calling the function:

<?php

  $img = imagecreate(1, 1);

  print_r([$img, $img ? 'TRUE': 'FALSE', is_resource($img) ? 'TRUE' : 'FALSE', get_resource_type($img) ?: 'FALSE']);

  /*
  Result:
  Array
  (
    [0] => Resource id #1
    [1] => TRUE
    [2] => TRUE
    [3] => gd

  )
  */

  imagedestroy($img);

  print_r([$img, $img ? 'TRUE': 'FALSE', is_resource($img) ? 'TRUE' : 'FALSE', get_resource_type($img) ?: 'FALSE']);

  /*
  Result:
  Array
  (
    [0] => Resource id #1
    [1] => TRUE
    [2] => FALSE
    [3] => Unknown
  )
  */
?>

As you can see in the above example, the first index in array is TRUE in both cases. So, despite the common thinking you can't trust in something like:

<?php

  if ($img)  {  // it's casted as boolean and returns true even after imagedestroy is called
     // do something
  }

?>

If you need to ensure the availability of a certain resource, you must to use is_resource and get_resource_type functions.
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