Haz algo como esto..
<?php
$json_data=$_POST['contactdetails'];
$data=json_decode($json_data, true); // Added true flag
// You can access your variables like this..
echo $data['phone number'];// prints "5555"
echo $data['username']; // prints "xfg"
//do your db connection...
// execute your query with those variables...
?>
