Rot 5 Caesar Ciphers for numbers - PHP Snippet

ROT5 covers the numbers 0-9.

function rot5($string){ 
for ($i = 0, $j = strlen( $string); $i < $j; $i++)  
{     
    $char = ord($string[$i]);      
    if( ($char >= 48  && $char <= 57))  
    { 
        $char += 5;          
        if( $char > 57) 
        { 
            $char -= 10; 
        } 
    }     
    $string[$i] = chr($char); 
}  
 return $string; 
}