<?
function phi($x) {
# constants
$a1 = 0.254829592;
$a2 = -0.284496736;
$a3 = 1.421413741;
$a4 = -1.453152027;
$a5 = 1.061405429;
$p = 0.3275911;
# Save the sign of x
$sign = 1;
if ($x < 0) {
$sign = -1;
}
$x = abs($x)/sqrt(2.0);
# A&S formula 7.1.26
$t = 1.0/(1.0 + $p*$x);
$y = 1.0 - ((((($a5*$t + $a4)*$t) + $a3)*$t + $a2)*$t + $a1)*$t*exp(-$x*$x);
return 0.5*(1.0 + $sign*$y);
}
?>