Here's a quick test using PHP 5.6. Note the differences of raising to a power if you're using an older version of PHP.
As I mentioned earlier, it could easily be simplified by pre-calculating the real numbers, but I left them as-is so they match the source formula.
<?php
# This has not been extensively tested!
#
# This uses the formula presented at:
# http://www.srh.noaa.gov/images/epz/wxcalc/altimeterSetting.pdf
#
# $p is absolute pressure in millibars, $h is station elevation in meters
#
$p = 933;
$h = 323.4;
# If you're using php 5.6 or better, you can use the ** power notation.
# Should the line below error-out, you probably have an older version of php,
# Comment it and the associated echo line and it should run.
$alt = ($p - 0.3)*(1+((1013.25**0.190284*0.0065/288)*($h/($p-0.3)**0.190284)))**(1/0.190284);
# If you're using an older version of php, you'll have to use the uglier pow()
# function but it has the advantage of working on more installations of php.
$alt2 = ($p - 0.3)*pow((1+((pow(1013.25,0.190284)*0.0065/288)*($h/pow(($p-0.3),0.190284)))),(1/0.190284));
echo "\n";
echo "Absolute Pressure in mb = $p\n";
echo "Station Elevation in meters = $h\n";
echo "Calculated Altimeter Setting using ** = $alt\n";
echo "Calculated Altimeter Setting using pow() = $alt2\n";
echo "\n";
?>
