Limit php if statement to evaluate last write

I'm trying to get a couple of php if statements to only run when the last fwrite has run. Here's my code snippet:

$cnt = 1;
fwrite($fh, $phptag);
$cnt = 2;
// The last fwrite that I want checked
fwrite($fh, $key);
// Check if write has been successful
if ($fwrite !== false && $cnt == 2) {
  $output = json_encode(array( // create JSON data
    'type'=>'success', 
    'text' => 'Congratulations !  The salt key has been successfully created.'
  ));
  exit($output); // exit script outputting json data
}
// Check if write has been unsuccessful
if ($fwrite === false && $cnt == 2) {
  $output = json_encode(array( // create JSON data
   'type'=>'error', 
    'text' => 'There has been an error in creating the salt key.'
  ));
  exit($output); // exit script outputting json data
}

I tried using $cnt to control when to run, but the $output is never sent back to the ajax done statement - just getting undefined. What did I do wrong?