<?php
$re = '~(.*time\s+)(\d{2}:\d{2}\s[ap]m)(.*)~';
$str = "there is some text and maybe some number or symbols time 12:32 am and there is some text";
$new_time = "10:10 pm";
$result = preg_replace($re, '${1}' . $new_time . "$3", $str); echo $result;
PD9waHAKCiRyZSA9ICd+KC4qdGltZVxzKykoXGR7Mn06XGR7Mn1cc1thcF1tKSguKil+JzsgCiRzdHIgPSAidGhlcmUgaXMgc29tZSB0ZXh0IGFuZCBtYXliZSBzb21lIG51bWJlciBvciBzeW1ib2xzIHRpbWUgMTI6MzIgYW0gYW5kIHRoZXJlIGlzIHNvbWUgdGV4dCI7IAokbmV3X3RpbWUgPSAiMTA6MTAgcG0iOyAKJHJlc3VsdCA9IHByZWdfcmVwbGFjZSgkcmUsICckezF9JyAuICRuZXdfdGltZSAuICIkMyIsICRzdHIpOwplY2hvICRyZXN1bHQ7