To build on Zak's answer:
You can also use PHP to echo out things that aren't PHP... as long as you quote it appropriately.
<?php
//HTML
while ($x < 5) {
echo "<p> this is html that you can wrap with html tags! </p>";
$x++;
}
//Javascript
echo "<script type='text/javascript'>
some javascript code
</script>"
?>
Although, it's less confusing to just end the php tag to keep things separate.
And you can even use php as you want within html or javascript as long as you put the tags, and as long as the file is saved as a .php file (so PHP can be processed on the server).
Ex:
<script type="text/javascript">
//set a javascript image array to a php value
var imgArray = [<?php echo implode(',', getImages()) ?>];
</script>
But if you want to do this the other way around (IE, assign a browser-compiled value, such as a javascript value to a php value), you'll need to use AJAX.