In recent versions of php, CURLOPT_MUTE has (probably) been deprecated. Any attempt of using curl_setopt() to set CURLOPT_MUTE will give you a warning like this:

PHP Notice:  Use of undefined constant CURLOPT_MUTE - assumed 'CURLOPT_MUTE' in ....

If you wish tu silence the curl output, use the following instead:

<?php
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
?>

And then,

<?php
    $curl_output=curl_exec($ch);
?>

The output of the curl operation will be stored as a string in $curl_output while the operation remains totally silent.