<?php

function salinity($measuredConductivity, $measuredTemperature, $measuredDepth) {

// PHP version by Ben Ostrowsky
// Based on Excel formulas by James Douglass
//   http://jimbodouglass.blogspot.com/2010/11/conductivity-to-salinity-conversion-for.html
// Based on Fofonoff, N.P. and Millard, R.C., Jr. (1983). Algorithms for computation
//   of fundamental properties of seawater. http://hdl.handle.net/1912/2470
// Added code to compensate for depth.

        $referenceConductivity = 42900.0; // microsiemens per cm^2

        $conductivityRatio = $measuredConductivity / $referenceConductivity;

        $waterDensity = 1.03 * 10^3; // kilograms per m^3 (pure water would be 1.00 * 10^3)
        $gravity = 9.8; // meters per s^2
        $airPressure = 10.0; // decibars

        $computedPressure =  ($waterDensity * $gravity * $measuredDepth / 10.0) + $airPressure;

        $littlert = (
                    0.6766097
                +   0.0200564     * $measuredTemperature
                +   0.0001104259  * ($measuredTemperature^2)
                + (-6.9698*10^-7) * ($measuredTemperature^3)
                + ( 1.0031*10^-9) * ($measuredTemperature^4)
        );

        $Rp =
                1 + (
                        (
                                $computedPressure * (
                                        2.070*10^-5   * ($computedPressure^0)
                                        + (-6.370*10^-10) * ($computedPressure^1)
                                        +   3.989*10^-15  * ($computedPressure^2)
                                )

                                /

                                (
                                        1
                                        + (3.426*10^-2) * ($measuredTemperature^1)
                                        + (4.464*10^-4) * ($measuredTemperature^2)
                                        + ( 4.215*10^-1 + (-3.107*10^-3) * $measuredTemperature ) * $conductivityRatio
                                )
                        )
                );

        $bigRt =  $conductivityRatio / ($littlert * $Rp);

        $dS =
                (
                        ( $measuredTemperature - 15.0) / (1 + 0.0162*( $measuredTemperature - 15.0 ) )
                )
                * (
                            0.0005
                        + (-0.0056) * ($bigRt^0.5)
                        + (-0.0066) * ($bigRt^1)
                        + (-0.0375) * ($bigRt^1.5)
                        + ( 0.0636) * ($bigRt^2)
                        + (-0.0144) * ($bigRt^2.5)
                );

        $computedSalinity =
                    0.008
                + (-0.1692) * ($bigRt^0.5)
                +  25.3851  * ($bigRt^1)
                +  14.0941  * ($bigRt^1.5)
                + (-7.0261) * ($bigRt^2)
                +   2.7081  * ($bigRt^2.5)
                + $dS;

        return $computedSalinity;

}

?>