php - htmlentities to allow <a> links - How? -

in order make inputs safe, i'm using htmlentities in php:

$input = $_post['field']; $result = htmlspecialchars($input); 

this works, realized in inputs, need allow basic markup <b> , <i>, copyright logos , basic stuff user. started doing this:

$result = $_post['ftext']; $presanitize = htmlspecialchars($result); $newftext = str_replace(array("&lt;i&gt;", "&lt;b&gt;", "&lt;/i&gt;", "&lt;/b&gt;", "&copy;", "&quot;", "&lt;a&gt;", "&lt;&#47;a&gt;"),  array("<i>", "<b>", "</i>", "</b>", "©", '"', "<a>", "</a>"), $presanitize);  

now come main problem: how allow things <a> , <img> don't have tag , don't know comes inside of it?

i can replace , because it's , if replace , wont work i'll have lots of stuff (<a href="http://link.com">text</a>) inside of it. should do? in advance.

the simple answer is: don't. that's part of reason why many popular forum systems use kind of markup that's not plain html. otherwise people can , nasty stuff way or another.

<img src="http://example.com/random-pic.jpg" onload="location.href='http://some.nasty.page/exploit';"/> 

but can remove event tags? of course, keep date browsers support , quirks? can outsmart everyone?

if still want it, documented, tested, , used library or script provides functionality. php has built in, it's barebone. keywords "php html sanitizer" or similar.

personally i'd recommend support markdown or bbcode syntax (again: there many ready use snippets , libraries available). don't reinvent wheel unless have to.