Parsing Conditions? Eval?

[Adrian Schneider]

I have a standalone system where users enter HTML, and I want to have some conditions in it (such as hide left block if that option is enabled). Basically in the HTML they will have something like this:

<table align="center" cellpadding="0" cellspacing="0" width="760">
	<tr valign="top">
		{IFSHOWLEFTBLOCK}
		<td width="{LEFTBLOCKWIDTH}" class="leftblock">
{LEFTBLOCKS}
		</td>
		<td width="{LEFTBLOCKSPACERWIDTH}"><img src="{SPACERIMAGE}" width="{LEFTBLOCKSPACERWIDTH}" alt="" /></td>
		{/IFSHOWLEFTBLOCK}


		<td class="content">
{BLOGPOSTBIT}
		</td>


		{IFSHOWRIGHTBLOCK}
		<td width="{RIGHTBLOCKSPACERWIDTH}"><img src="{SPACERIMAGE}" alt="" width="{RIGHTBLOCKSPACERWIDTH}"/></td>
		<td width="{RIGHTBLOCKWIDTH}" class="rightblock">
{RIGHTBLOCKS}
		</td>
		{/IFSHOWRIGHTBLOCK}


	</tr>
</table>

Right now I am just using preg_replace to replace the {variables} with the corresponding $variable (preset ones, they can't use anything), and I want the same for conditions.

I'm thinking eval (but I've never used much of it before), so how would I go about doing this with the IFs? Right now I am using preg replace, so if the condition they enter is false, it replaces the condtiions with <!-- and --> (very cheap solution) and if true, replaces with "".

I do not want this like vBulletins, where it runs any PHP they enter in the conditions, only presets.