20180211.php

<?php
// https://www.facebook.com/groups/2204685680/permalink/10156550592900681/

// These are your inputs
$str1 = "We all go to college";
$str2 = "We all go to temple";

// Create array of lowercase versions of the words
// Lowercase to allow our diff to not get caught up in case sensitivity
$ar1 = array_filter(array_map('trim', explode(' ', strtolower($str1))));
$ar2 = array_filter(array_map('trim', explode(' ', strtolower($str2))));

// Now we get an array of all the words which only appear in either of the arrays
$diff = array_merge(
    array_diff($ar1, $ar2),
    array_diff($ar2, $ar1)
);

// This is what we'll replace the differences with
$replacePattern = '<span class="highlight">${1}</span>';

$str1_highlighted = $str1;
$str2_highlighted = $str2;
foreach ($diff as $part) {
    $str1_highlighted = preg_replace(
        sprintf('/(%s)/i', preg_quote($part)), 
        $replacePattern,
        $str1_highlighted
    );
    $str2_highlighted = preg_replace(
      sprintf('/(%s)/i', preg_quote($part)), 
        $replacePattern,
        $str2_highlighted
    );  
}

// Now, $str1_highlighted and $str2_highlighted can be output as desired