I need to calculate the Vapor Pressure Deficit (VPD) using a php function and the given is temprature in fahrenheit and relative humidity as a whole number so if the relative humidity is 60% the $humidity variable equals 60. This is what I have but its not producing the correct value.

Im using the formula from:

https://betterorganix.com/blog/what-is-how-to-calculate-vapour-pressure-deficit/

My code is:

function VPD($temp,$humidity) { $p1=(7.5*$temp); $p2=(237.3+$temp); $power=$p1/$p2;

$SVP = (610.7*10^$power)/1000; $VPD = ((100 - $humidity)/100)*$SVP;

return $VPD; }

echo(VPD(21.5,49.0));