[php] How to check if a MySQL query using the legacy API was successful?

If your query failed, you'll receive a FALSE return value. Otherwise you'll receive a resource/TRUE.

$result = mysql_query($query);

if(!$result){
    /* check for error, die, etc */
}

Basically as long as it's not false, you're fine. Afterwards, you can continue your code.

if(!$result)

This part of the code actually runs your query.