I need to know the date for last Sunday for given date in php & javascript

Let's have a function give_me_last_Sunday

give_me_last_Sunday('20110517') is 20110515
give_me_last_Sunday('20110604') is 20110529

The full backup is done on Sundays = weekly. If I want to restore daily backup I need full (weekly) and daily backup. I need to copy backup files before restoring to temp directory so I restoring daily backup I need to know what weekly backup file I need to copy along the daily file.

My thought was to get Julian representation (or something similar) for the given date and then subtract 1 and check if it is Sunday ... Not sure if this is the best idea and how to convert given date into something I can subtract.

javascript datetime
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