怎么劈开一组数字中的4与7

$strMaxcardid=3655448674;
$ar = str_split($strMaxcardid);

		foreach($ar as $k => $v) {
			
    	if ($v == 4 || $v == 7) { 
           $ar[$v] ++;       
            break;    
        }}
  		$str = join('', $ar);
  		echo $str;*/

$strMaxcardid=3655448674;
$ar = str_split($strMaxcardid);
 
foreach($ar as $k => $v) {
  if ($v == 4 
------解决思路----------------------
 $v == 7) { 
    $ar[$k] ++;       
  }
}
$str = join('', $ar);
echo $str;