How to gracefully handle files that exceed PHP's `post_max_size`?

From the documentation :

If the size of post data is greater than post_max_size, the $_POST and $_FILES superglobals are empty. This can be tracked in various ways, e.g. by passing the $_GET variable to the script processing the data, i.e. <form action="edit.php?processed=1">, and then checking if $_GET['processed'] is set.

So unfortunately, it doesn't look like PHP sends an error. And since it sends am empty $_POST array, that is why your script is going back to the blank form - it doesn't think it is a POST. (Quite a poor design decision IMHO)

This commenter also has an interesting idea.

It seems that a more elegant way is comparison between post_max_size and $_SERVER['CONTENT_LENGTH']. Please note that the latter includes not only size of uploaded file plus post data but also multipart sequences.

I had the same problem, and combined some of the solutions already posted here on this page (by @Doblas, @Lance Cleveland and @AbdullahAJM).

Additionally, my solution tries to sends a 413 Payload Too Large error (instead of 200 OK), which is of course only possible, when php.ini is not configured to display warnings.

// Check for Warning: php catch Warning: Unknown: POST Content-Length of bytes exceeds the limit of bytes in Unknown on line 0
// Sending 413 only works, if Warnings are turned off in php.ini!!!

// grab the size limits...
$postMaxSize = trim(ini_get('post_max_size')); 
if (strlen($postMaxSize)>0) {
   $postMaxSizeValue = substr($postMaxSize, 0, -1);
   $postMaxSizeUnit = strtolower(substr($postMaxSize, -1));
   $postMaxSize = 0; // make it fail save
   if (false !== filter_var($postMaxSizeValue, FILTER_VALIDATE_INT, array('options' => array( 'min_range' => 0)))) {
      switch ($postMaxSizeUnit) {
         case 'g': $postMaxSizeValue*=1024; // ... and fall through
         case 'm': $postMaxSizeValue*=1024; // ... and fall through
         case 'k': $postMaxSizeValue*=1024; break;
         default: if ($postMaxSizeUnit>='0' && $postMaxSizeUnit<='9') {
                     $postMaxSizeValue = (int) $postMaxSizeValue.$postMaxSizeUnit;
                  } else {
                     $postMaxSizeValue = 0;
                  }
      }
      $postMaxSize = $postMaxSizeValue;
   }
} else {
   $postMaxSize = 0;
}

if (empty($_FILES) && empty($_POST) &&
    isset($_SERVER['REQUEST_METHOD']) && $_SERVER['REQUEST_METHOD'] == 'POST' &&
    isset($_SERVER['CONTENT_LENGTH']) && $_SERVER['CONTENT_LENGTH'] > $postMaxSize) {
    // if is a post request and $_POST variable is empty(a symptom of "post max size error")
    
    if (headers_sent()) {
       // echo out error and solutions...
       echo "<p style=\"color: #F00;\">\nPlease note that an error <b>413 Payload Too Large</b> should be sent, but the warning can't be catched, and so the client gets a <b>200 OK</b>. ".
            "Please turn off warnings in php.ini in order to achieve the correct behaviour.</p>"; 
    } else {
       http_response_code(413);
    }

    // catch file overload error: echo out error and solutions...
    echo "<p style=\"color: #F00;\">\nPlease note files larger than ".$postMaxSize." will result in this error!<br>".
         "Please be advised this is not a limitation in the script, this is a limitation of the hosting server.</p>";
    exit(1);
}