Datenbank: DELETE FROM ... klappt nicht

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<form name=\"delete_posteingang\" action=\"massager.php\" method=\"POST\"><td width=\"20\"><input type=\"image\" name=\"delete_posteingang\" class=\"none\" onClick=\"delmsg(); return false;\" img src=\"msgdel.gif\" width=\"13\" height=\"14\" border=\"0\" align=\"absmiddle\" title=\"L&ouml;schen\"></td></form>

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if (isset($_POST['delete_posteingang'])) {
mysql_query("DELETE FROM posteingang WHERE id = '$z' LIMIT 1");
}

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$result = mysql_query("SELECT * FROM posteingang WHERE absender = '$username'");
while ($row = mysql_fetch_array($result)) {

$num_rows = mysql_num_rows($result);

$z = $row['id'];

}