PHP div problem

[kerrynet]

I am writing my first php code and have run into a spacing problem. I have displaying some div's that are in columns and are not equals heights. Everything was fine until near the end of the template. see http://www.s-v-g.co.uk/php_site/test.php I have been trying to move the bottom left grey div up to rest under the left red menu, but it will not move up past the yellow middle content div. Can you have a look at the sourse code to see where I have gone wrong.

#sidebar {
width:150px;
height:600px;
float:left;
margin-bottom:0px;
border-left:1px solid #ccc;
background-color:red;
 
}

#sidebar a {
text-decoration:none;
}

#sidebar li {
list-style:none;
}

#rightbar {
width:160px;
height:600px;
float:right;
margin-bottom:0px;
border-left:1px solid #ccc;
background-color:lightgreen;
}

#leftlowerad {
width:150px;
height:300px;
float:left;
margin-bottom:0px;
border-left:0px solid #ccc;
background-color:lightblue;
 

}

#rightlowerad {
width:160px;
height:300px;
float:right;
margin-bottom:0px;
border-left:0px solid #ccc;
background-color:lightblue;
}

#footer {
clear:both;
width:1038px;
height:135px;
border-top:1px solid #ccc;