int mysql_errno([resource $res])

(PHP 4, PHP 5)

Returns the error code for the last MySQL error generated. mysql_query function will return false if an error occurred. You can check for this when running your query and use mysql_errno to get the error code. If the optional parameter $res is left out, the function will use the last database resource opened by the script.

// open the mysql resource
$res = mysql_connect($server, $user, $pass);
 
// insert a single row into tblState
if (!mysql_query('INSERT * INTO tblState (\'California\')', $res)) {
    // get the error number
    $errno = mysql_errno($res);
}