This function calculates percentile value for given data set and percentile. Similar to MS excel =PERCENTILE(data,k) function.

percentile_as_excel.php

<?php
function percentile_as_excel($data, $percentile){
	if( 0 <= $percentile && $percentile <= 1 ) {
		$p = $percentile;
	}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;
}