There is actually a php function called strip_tags();
http://www.php.net/manual/en/function.strip-tags.php
Also you could use ereg_replace:
http://www.php.net/manual/en/function.ereg-replace.php
http://www.php.net/manual/en/function.preg-replace.php
Assuming $text was the email put into a variable:
$search = array ("'<script[^>]*?>.*?</script>'si", // Strip out javascript
"'<[\/\!]*?[^<>]*?>'si", // Strip out html tags
"'([\r\n])[\s]+'", // Strip out white space
"'&(quot|#34);'i", // Replace html entities
"'&(amp|#38);'i",
"'&(lt|#60);'i",
"'&(gt|#62);'i",
"'&(nbsp|#160);'i",
"'&(iexcl|#161);'i",
"'&(cent|#162);'i",
"'&(pound|#163);'i",
"'&(copy|#169);'i",
"'&#(\d+);'e"); // evaluate as php
$replace = array ("",
"",
"\\1",
"\"",
"&",
"<",
">",
" ",
chr(161),
chr(162),
chr(163),
chr(169),
"chr(\\1)");
$text = preg_replace ($search, $replace, $text);