For the timezone Europe/London on date 2018-10-28 two hour 01:00:00 exist - one DST and another non-DST. Following code shows that it gives two different timestamp based on the set default timezone.

PHP 8.1.5 crashes on strtotime() with a SIGSEGV if the file associated with the selected timezone or the fallback (UTC) is not readable at the expected location, e.g. if it is not exposed to a chroot.