Description:
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The preg_replace function returns a NULL value on a not NULL input string. Even if there is no problem with backtrack_limit and recursion_limit.
The preg_last_error() returns 0.

Reproduce code:
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$search = array("s1"=>"s1");   //In reality the arrays have 3000 entries
$repl = array("s1"=>"S1");
$str = "bla";
$str = preg_replace($search,$repl,$str);
echo $str;

Expected result:
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bla

Actual result:
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NULL