PHP | JS: convert date format to sql timestamp

HTML
<input id="myinput" type="text" value="20/09/1973" /> <!-- format: dd/mm/yyyy -->


Javascript
var mydate = document.getElementById("myinput").value;
var t = mydate.replace(/^(\d{2})\/(\d{2})\/(\d{4})$/, "$3-$2-$1 00:00:00");

//t= "1973-09-20 00:00:00"



PHP
echo preg_replace("/^(\d{2})\/(\d{2})\/(\d{4})$/", "$3-$2-$1 00:00:00", "20/09/1973");