Description:
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PCRE allows literal matches of strings between \Q and \E. This is also documented, \Q.$.\E will match literal .$.

However, if that literal string contains the regexp delimiter (/ or # or ~ or () or whichever you choose), the regexp compile either fails, or the match fails because it tries to match the escape char used to escape the delimiter.

The problem is php_pcre::pcre_get_compiled_regex_cache() which parses the delimiter, not taking \Q \E in account. Delimiters between \Q \E should be treated as literal characters, not delimiters (that's what Perl does); or alternatively if delimiters have to be escaped, the escape char should be removed from the pattern.

Workaround: Use preg_quote() instead of \Q \E if there's a chance the delimiter may appear within \Q \E

Test script:
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preg_replace("/\Q/#~\E/", ...);
=> Warning: preg_replace(): Unknown modifier '#' in php shell code on line 1

preg_replace("/\Q\/#~\E/", "OK", "/#~");
=> "/#~" (expected "OK")

preg_replace("/\Q\/#~\E/", "FAIL", "\/#~")
=> "FAIL" (expected "\/#~");