<?php

function some_function($a, $b)
{
    $result = '';
    $len = strlen($a) > strlen($b) ? strlen($b) : strlen($a);
    for($i=0; $i<$len; $i++)
    {
        if(substr($a, $i, 1) == substr($b, $i, 1))
        {
            $result .= substr($a, $i, 1);
        }
        else
        {
            break;
        }
    }
    return $result;
}

$a = 'Metric Bob Foundation A5, Lined, Pink &amp; Orange';
$b = 'Metric Bob Foundation A5, Lined, Navy &amp; Green';
echo some_function($a, $b);

Metric Bob Foundation A5, Lined,