รบกวน ด้วย คับ ขอคำแนะนำ การ คูณ 11 5/8" x 15 cm

function str2exp($string){
echo $string."<br />\n";
	$multi = array('x','X');
	$str_exp = str_replace("\"","x2.54",$string);
$str_exp = str_replace("cm","",$str_exp);
$str_exp = str_replace($multi,"x",$str_exp);
$str_exp = str_replace(" ","x",$str_exp);
echo $str_exp;	
$expression = explode("x",$str_exp);
$expression = array_filter($expression);
print_r($expression);
$result=1;
foreach($expression as $val){
	if(strstr($val,"/")){
		list($a,$b) = explode("/",$val);
		$result *= ($a/$b);
	}else{
		$result *= $val;
	}
}
return $result;
}

$quiz = array('11" x 15"','11 x 15cm','11 5/8" x 15 cm','11" x 15 cm','11"');
foreach($quiz as $each){
	echo str2exp($each)."<hr />";
}

// output 
/*
11" x 15"
11x2.54xxx15x2.54Array ( [0] => 11 [1] => 2.54 [4] => 15 [5] => 2.54 ) 1064.514

11 x 15cm
11xxx15Array ( [0] => 11 [3] => 15 ) 165

11 5/8" x 15 cm
11x5/8x2.54xxx15xArray ( [0] => 11 [1] => 5/8 [2] => 2.54 [5] => 15 ) 261.9375 // ตัวนี้ คับ ที่ติด

11" x 15 cm
11x2.54xxx15xArray ( [0] => 11 [1] => 2.54 [4] => 15 ) 419.1

11"
11x2.54Array ( [0] => 11 [1] => 2.54 ) 27.94
*/