Chi Square Probability Distribution Code Using PHP

This example is a Web application piece of code that I wrote to add a (approximate) p-value to some dynamically generated crosstabs. This will allow a researcher to provide a way to deliver data over the web and will allow the researcher a way to calculate the p-value from a $\chi^{2}$ distribution based on input data. In order to use this function the researcher must calculate the $\chi^{2}$ value first by using the standard $\chi^{2}$ equation: $\sum \frac{ \left( O - E \right)^{2} }{E}$ . In addition to the $\chi^{2}$ statistic the researcher needs to provide the degrees of freedom.

function getChiSquare($x, $n) {
if ( ($n==1) && ($x > 1000) ) {
return 0;
}

if ( ($x>1000) || ($n>1000) ) {
$q = getChiSquare(($x-$n)*($x-$n)/(2*$n),1) / 2;
if($x > $n) {
return $q;
} else {
return 1 - $q;
}
}
$p = exp(-0.5 * $x);
if(($n % 2) == 1) {
$p = $p * sqrt(2*$x/pi());
}
$k = $n;
while($k >= 2) {
$p = $p * ($x/$k);
$k = $k - 2;
}
$t = $p;
$a = $n;
while($t > 0.0000000001 * $p) {
$a = $a + 2;
$t = $t * ($x / $a);
$p = $p + $t;
}

$retval = 1-$p;
return $retval;
}