Description:
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If pattern and replace are single element arrays and the keys are non-zero integers, then preg_replace may modify the replace parameter.  In the case of the test script, the replace parameter has an empty string inserted as the 0th entry after the call to preg_replace.  

I have not tested string keys or with more than one array element.  

Test script:
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https://gist.github.com/pglewis/42c7a6e3da5e465793e0

Expected result:
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I could find no caveats regarding keys in the documentation, so the expectation is that the replace parameter should not be modified in any way by a call to preg_replace.  

Actual result:
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An empty string is inserted into the replace parameter as an extra array element, resulting in an unexpected result and breaking backward compatibility.