php - SELECT WHERE Date is equal to what user typed not working -

i'm trying user type date , make php display records who's date (date type field) equal date user entered. keep getting 0 results when should @ least show 1.

here's current code. note had fill in case user leaves date text box empty (in case doesn't want search date)

$query = 'select fname,lastname recordsdb joindate = ?';

and section:

$bind_fdate = date("y-m-d"); if ($_post["userinputdate"] != "enterdatehere") {$bind_fdate = strtotime($_post["userinputdate"]);} mysqli_stmt_bind_param($stmt,"s",$bind_fdate); 

i showing needed parts code works until added date comparison part of it. need display records who's dates matches date user types in. joindate field in database date type field (yyyy-mm-dd) 2015-07-21. there's no date type in mysqli_stmt_bind_param used s. if user types 2015-07-21 or 2015-7-21 0 result. doing wrong?

could problem related date_format

 "select fname,lastname recordsdb joindate = date_format(? , '%y-%m-%d')";