<?php
$x = 'a:1:{s:10:"guest_name"";s:10:""Kelly Gallagher"";}"';

$x = preg_replace('/s:([1-9][0-9]*):""/', 's:${1}:"', $x);
$x = preg_replace('/(\w)"";/', '${1}";', $x);
$x = preg_replace('/"$/', '', $x);

echo "$x\n";