Canceling open tags and User submitted HTML handling

I am creating a user input interface (similar to this vB post box I’m typing in now), but am not sure of the best way to handle HTML input.

I would like to allow users the opportunity to use some formating tags, such as <em>, <strong>, <a>, and <blockquote>, and can accomplish this using:

[code=php]$Text = strip_tags($Text, ‘<em><strong><blockquote><a>’);[/code]

But how do I ensure that any opened tags are closed after a user’s post, and can’t be left open and “bleed” onto the rest of the page?

I could close the above tags just in case like:

[code=php]$Text .= “nr</em></strong></a></blockquote>”;[/code]

but that isn’t [I]really[/I] valid, since I’m closing tags that might not be open.

How does vB / PHPBB (and others) handle this? Or what’s the best way of ensuring that open user-allowed tags get closed?

Thanks for everything,

Nick

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