title: Get first day date of the week on a given date PHP
body: <p>I have a date <code>2015-12-16</code></p>
  
  <p>i want to get the date of the first day's date for the current week of my date</p>
  
  <p>here <code>2015-12-16</code> is in the week 51 of the year then i want to get the first day's date of the week 51 ( <code>2015-12-14</code> here)</p>
  
  <p>how could i do it ?
  thank you</p>
  
  <p>EDIT: it must work when there are 53 weeks in the year (like in 2015 for example)</p>