If you are looking to infer the percentile from a z-score, you can use this function.
It's far from precise but does the job on most circumstances.
The error function ( erf() )is based on the approximation on wikipedia: 
http://en.wikipedia.org/wiki/Error_function

<?php
function erf($x)
{
        $pi = 3.1415927;
        $a = (8*($pi - 3))/(3*$pi*(4 - $pi));
        $x2 = $x * $x;

        $ax2 = $a * $x2;
        $num = (4/$pi) + $ax2;
        $denom = 1 + $ax2;

        $inner = (-$x2)*$num/$denom;
        $erf2 = 1 - exp($inner);

        return sqrt($erf2);
}

function cdf($n)
{
        if($n < 0)
        {
                return (1 - erf($n / sqrt(2)))/2;
        }
        else
        {
                return (1 + erf($n / sqrt(2)))/2;
        }
}

// EXAMPLE
$sample = 90;
$avg = 75;
$stddev = 12;

$zscore = ($sample - $avg) / $stddev;
print 'Percentile: ' . cdf($zscore) * 100 . "\n";
?>

Where $n is the z-score

The function cdf() returns the approximed cumulative standard normal distribution ([0..1])



[EDIT BY danbrown AT php DOT net: Contains a bugfix provided by (Ed) on 24-FEB-2010 which fixes the definition of $a in erf() with the note that it is "out by a factor of -1" in the original code.]

I have looked at the code available in the Math package for percentile. I compared the result obtained with percentile from Excel and found that it doesnt match. So i wrote my own percentile function and verified the result with the Excel's percentile.

For those of you who need the percentile calculation of Excel in php..

<?php
function mypercentile($data,$percentile){
    if( 0 < $percentile && $percentile < 1 ) {
        $p = $percentile;
    }else if( 1 < $percentile && $percentile <= 100 ) {
        $p = $percentile * .01;
    }else {
        return "";
    }
    $count = count($data);
    $allindex = ($count-1)*$p;
    $intvalindex = intval($allindex);
    $floatval = $allindex - $intvalindex;
    sort($data);
    if(!is_float($floatval)){
        $result = $data[$intvalindex];
    }else {
        if($count > $intvalindex+1)
            $result = $floatval*($data[$intvalindex+1] - $data[$intvalindex]) + $data[$intvalindex];
        else
            $result = $data[$intvalindex];
    }
    return $result;
}
?>

The above code may not be elegant.. but it solves my problem..

yuvaraj