/*
    - Temperature Conversion
    - Converts one temperature to another
   
    $from     - The temperature to convert from.
                => "c" - celsius
                => "f" - fahrenheit
                => "k" - kelvin
   
    $to       - The temperature to convert to.
                => "c" - celsius
                => "f" - fahrenheit
                => "k" - kelvin
   
    $value    - The temperature to be converted
    */
 
    function convertTemperature($from, $to, $value)
    {
        $from = strtolower($from);
        $to = strtolower($to);
 
        if(!in_array($from, array("f", "c", "k") )) die("Invalid From value");
        if(!in_array($to, array("f", "c", "k") )) die("Invalid To value");
       
        if (is_numeric($value) || is_double($value))
        {
            // Get input value in degree Kelvin, if($from == "K") then no action required.
            if($from == "c") $value += 273.15;
            if($from == "f") $value = (($value - 32) / 1.8) + 273.15;
            if($value<0) die("Invalid input value entered."); // Validating received value.
 
            // GET converted value, if($to == "K") // No acton required.
            if($to == "c") $value -= 273.15;
            if($to == "f") $value = (($value - 273.15) * 1.8) + 32;
 
            return $value;
        }
        else
        {
            return false;
        }
    }
 
// for example.
    echo convertTemperature("c", "f", 39);