Functions to escape CSS rules in PHP
There's not much to say. Overall, the code is small and quite readable.

Just a small nitpick:

if (getUTF8CharCode($currChar) &lt; 256 &amp;&amp; //Character value is less than 256
    !preg_match("/^\w$/", $currChar) &amp;&amp; //Character is not alphanumeric (underscore is considered safe too)
    !in_array($currChar, $immuneChars)  //Character is not immune


These comments say the what, not the why. They're akin to:

$i++; // increments i by one


Your if condition should be commented, but not like this.

I can't comment on the security-side of things.