Description:
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Strtotime() function creates problem when we convert a date which is in afternoon time. 

Please see the example below. I have tried in 5.3.5 version of PHP on Wamp Server 2.

//it returns just empty
echo strtotime('Sep 23 2011 16:55PM');

// it returns as intended
echo strtotime('Sep 23 2011 04:55PM');

Test script:
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echo strtotime('Sep 23 2011 16:55PM');