Description:
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Sometime,the date('W') result is incorrect.

Test script:
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var_dump(date('Y-m-d H:i:s, W', mktime(0,0,0,12,31,2014)));
var_dump(date('Y-m-d H:i:s, W', mktime(0,0,0,1,1,2015)));

Expected result:
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string '2014-12-31 00:00:00, 53' (length=23)
string '2015-01-01 00:00:00, 53' (length=23)

Actual result:
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string '2014-12-31 00:00:00, 01' (length=23)
string '2015-01-01 00:00:00, 01' (length=23)