Solution

<?php
// Fixing the first number
$array1['7787'] = intval(md5(7787));    //Array because if you want to try multiple nos

for ($i = 7777777777; $i < 9999999999; $i++)
{
    if (!preg_match("!^".str_repeat("[7-9]",10)."$!", $i))
        continue;
    else
    {
        $num2hash = md5($i); $value = intval($num2hash);
        if ($value <= 0) continue;
        if ($num1 = array_search($value, $array1))
        {
            $num1hash = md5($num1);
            if ($num1hash[1] != $num2hash[0])
                echo "First is : ".array_search($value, $array1)."\nSecond is : $i\n";
        }
    }
}