Or:
<?php
echo "<center>This is html</center>";
?>
or even
<?php
print("<center>This is html</center>");
?>
Jim's way is cleaner; using these methods you have to remember to escape all " in the html:
print("<input type =\"text\" name = \"fieldname\">");
but this way allows you to access any variables you have declared without having to leap back into php in the middle of an html block.
Sheila