Is there any step to input a date and return the day of the current month

Is there any way to input a date and return the day of the current month? For example, I don't seem to find it. it's just a method like jddayofweek ------ solution ------------------ what day is the month? Date (d, strtotime ($ str); ------ solution ---------------- is there any way to pass in a date and return the day of the current month?
For example, I don't seem to find a question. it is a method similar to jddayofweek.

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The day of the month?
Date ('D', strtotime ($ str ));
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If you specify a date and return the day of the current month, then you can directly obtain the number of days. for example, 2013-1-5 is not the first day of January 5th?
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Date ('D', strtotime (date ))