Note that single-quotes around the parameter markers _will_ prevent your statement from being prepared correctly.
Ex:

<?php
$stmt = $mysqli->prepare("INSERT INTO City (District) VALUES ('?')");
echo $stmt->param_count." parameters\n";
?>
will print 0 and fail with "Number of variables doesn't match number of parameters in prepared statement" warning when you try to bind the variables to it.

But

<?php
$stmt = $mysqli->prepare("INSERT INTO City (District) VALUES (?)");
echo $stmt->param_count." parameters\n";
?>
will print 1 and function correctly.

Very annoying, took me an hour to figure this out.