Easy, explode them and then use array_diff:
$totalWords = count($array_1);
$array_1 = explode(" ", $str1);
$array_2 = explode(" ", $str2);
$differenceCount = count(array_diff($array_1, $array_2));
$differentPercent = $differenceCount / ($totalWords / 100);
@Edit:
Edited code above to display percentage. However remember you may have a wrong result if the word count of array 1 and array 2 is not identical.