PHP 7.0 added declare(strict_types=1);, which enables strict type checking of all method calls made in a file with that declaration, including internal functions such as strtotime(). https://wiki.php.net/rfc/scalar_type_hints_v5#strict_types_declare_directive

Timecop seems to be clobbering these type checks, resulting in no typechecking on calls made to overridden functions, as if strict_types wasn't enabled.

The following script, when run without timecop enabled, will fatally error "strtotime() expects parameter 1 to be string, null given" (as is correct). With timecop, the error is suppressed, causing execution to continue to show 'End!':

<?php
declare(strict_types=1);

strtotime(null); // fatal error without timecop, silent success with timecop
echo "End!\n";

This resulted in fatal errors being thrown in our Production environment, which do not occur in our Testing environment, as the latter uses Timecop. While those erroneous nulls should be patched, it was pretty confusing to trace down the in-production-only error, and I suspect this is likely to happen to others as the intended usage of Timecop is to be run in testing only.