I'm looking for a way to create valid UTF-16 JavaScript escape sequence characters (including surrogate pairs) from within PHP.

I'm using the code below to get the UTF-32 code points (from a UTF-8 encoded character). This works as JavaScript escape characters (eg. '\u00E1' for 'á') - until you get into the upper ranges where you get surrogate pairs (eg '??' comes out as '\u1D715' but should be '\uD835\uDF15')...

function toOrdinal($chr)
{
    if (ord($chr{0}) >= 0 && ord($chr{0}) <= 127) 
    {
        return ord($chr{0});
    }
    elseif (ord($chr{0}) >= 192 && ord($chr{0}) <= 223)
    {
        return (ord($chr{0}) - 192) * 64 + (ord($chr{1}) - 128);
    }
    elseif (ord($chr{0}) >= 224 && ord($chr{0}) <= 239)
    {
        return (ord($chr{0}) - 224) * 4096 + (ord($chr{1}) - 128) * 64 + (ord($chr{2}) - 128);
    }
    elseif (ord($chr{0}) >= 240 && ord($chr{0}) <= 247)
    {
        return (ord($chr{0}) - 240) * 262144 + (ord($chr{1}) - 128) * 4096 + (ord($chr{2}) - 128) * 64 + (ord($chr{3}) - 128);
    }
    elseif (ord($chr{0}) >= 248 && ord($chr{0}) <= 251)
    {
        return (ord($chr{0}) - 248) * 16777216 + (ord($chr{1}) - 128) * 262144 + (ord($chr{2}) - 128) * 4096 + (ord($chr{3}) - 128) * 64 + (ord($chr{4}) - 128);
    }
    elseif (ord($chr{0}) >= 252 && ord($chr{0}) <= 253)
    {
         return (ord($chr{0}) - 252) * 1073741824 + (ord($chr{1}) - 128) * 16777216 + (ord($chr{2}) - 128) * 262144 + (ord($chr{3}) - 128) * 4096 + (ord($chr{4}) - 128) * 64 + (ord($chr{5}) - 128);
    }
}

How do I adapt this code to give me proper UTF-16 code points? Thanks!