ISBN-10 Validation Written in PHP

ISBN-10

<?php
echo"Enter the ISBN number: ";
$isbn = trim(fgets(STDIN));
$isbn_length = strlen($isbn); //length
if (10 != $isbn_length) //ISBN length = 10?
  { echo "Not Valid";
    exit;
  }
$arr1 = str_split($isbn); // to array
 $m0 = $arr1[0] * 10;
 $m1 = $arr1[1] * 9;
 $m2 = $arr1[2] * 8;
 $m3 = $arr1[3] * 7;
 $m4 = $arr1[4] * 6;
 $m5 = $arr1[5] * 5;
 $m6 = $arr1[6] * 4;
 $m7 = $arr1[7] * 3;
 $m8 = $arr1[8] * 2;
 $m9 = $arr1[9] * 1;
 $total = $m0+$m1+$m2+$m3+$m4+$m5+$m6+$m7+$m8+$m9;
if($total %11 != 0)
  { echo "Not Valid"; 
    exit;
  }
  
else {echo "Valid";}
exit;
?>