How can I slash out a price to denote a sale price in html and php?

Well, the easiest way is to use the <strike> tag in html. This put a line through your text like so:
Code:


<strike>$9,999</strike>

Result:

$9,999

For a more complex solution, you can use PHP to create an image, on the fly, putting a red slash across your prices. Here's a solution:

<?php

header("Content-type: image/png");
$string = $_GET['n'];
$string = "$".number_format($string);
$im    = imagecreatefrompng("blank.png");
$black = imagecolorallocate($im, 0, 0, 0);
$red = imagecolorallocate($im, 196, 0, 0);
imagestring($im,5, 0, 0, $string, $black);
$points = array(0,11,1,13,54,6,55,4);
imagefilledpolygon($im,$points,4,$red);
imagepng($im);
imagepng($im,"./".$_GET['n'].".png");
imagedestroy($im);

?>

Note a few things:

This script gets called like so: script.php?n=8888, where n is the price you wish to display.
You need a blank (white, or whatever color your background is) image - called blank.png in the same directory as the php script.
The data points in the "array" line probably need tweaking, depending how big your image is. Do some testing, see what works.