, array() . , ? , , , . , .
:
isset($array[$key]) . , , $array[$key] null. array_key_exists($key, $array).
SQL, , :
$sqlvalues = array();
foreach( $options as $field ) {
if( array_key_exists('checkbox_'.$field, $_POST) )
$sqlvalues[] = $field.' = \'on\'';
else
$sqlvalues[] = $field.' = \'off\'';
}
mysql_query('UPDATE '.$table.' SET '.implode(', ', $sqlvalues).' WHERE applicant_id = '.$applicant_id);