Well here is what php is seeing:
$page_content=" <center>
<h1>The main content of the page.</h1>
<br>
<br>
<img src="
then something weird called:
http://www.nokia-gizmos.co.uk/addesign1.gif" width="
and
468" height="60"</center>";
first of all let me say I am very suprised you are not getting a parse error. PHP sees anything between " " as a string. Well in your image tags you have a lot of those, it doesnt really know what to do with em. We need to let PHP not that... hey... we want the symbol " and are not trying to make a new string here. How is that done? With the \" character. By placing a \ in front of the " we are telling PHP to escape trying to make what is inbetween the "" a string, and turn it into a literal. So you should read:
$page_content="
<center>
<h1>The main content of the page.</h1>
<br>
<br>
<img src=\"http://www.nokia-gizmos.co.uk/addesign1.gif\" width=\"468\" height=\"60\">
</center>";
Hope that helps!