First, you'll need to create an array of font names.
<?php
$fonts = array("arial", "verdana", "tahoma");
?>
Then you'll need to randomly select which to use in the font tag.
<font face="<?php echo $fonts[rand(0,2)]; ?>" size="<?php echo rand(5,10); ?>">Some Text</font>
That code will select a random font (arial is at index 0, verdana at index 1, etc..) and also select a random size from 5 to 10.