The four values required here are a bit confusing. After all, a circle is defined by three values: the x, y coordinates of the centre, and the radius, r.
The fourth value is redundant, but has to be given, otherwise the function fails. One way of coping with this redundancy is:
<?php
$draw = new ImagickDraw ();
$draw->circle ($x, $y, $x + $r, $y);
?>
There are any number of actions which are synonymous with the last, including:
<?php
$draw->circle ($x, $y, $x, $y + $r);
$draw->circle ($x, $y, $x - $r, $y);
$draw->circle ($x, $y, $x, $y - $r);
?>
Hope this helps.