Add kelvin to celcius conversion; as (liquid gas) coolant monitors report in Kelvin. (#15836)

includes/common.php

@@ -735,6 +735,23 @@ function celsius_to_fahrenheit($value, $scale = 'celsius')
     return sprintf('%.02f', $value);
 }
 
+/**
+ * Converts kelvin to fahrenheit (with 2 decimal places)
+ * if $scale is not celsius, it assumes celsius and  returns the value
+ *
+ * @param  float  $value
+ * @param  string  $scale  fahrenheit or celsius
+ * @return string (containing a float)
+ */
+function kelvin_to_celsius($value, $scale = 'celsius')
+{
+    if ($scale === 'celsius') {
+        $value = $value - 273.15;
+    }
+
+    return sprintf('%.02f', $value);
+}
+
 /**
  * Converts string to float
  */

includes/html/modal/new_customoid.inc.php

@@ -67,6 +67,7 @@ if (! (Auth::user()->hasGlobalAdmin())) {
                                 <option value=""></option>
                                 <option value="celsius_to_fahrenheit">C to F</option>
                                 <option value="fahrenheit_to_celsius">F to C</option>
+                                <option value="kelvin_to_celsius">K to C</option>
                                 <option value="uw_to_dbm">uW to dBm</option>
                             </select>
                         </div>

includes/polling/customoid.inc.php

@@ -15,6 +15,7 @@ foreach (dbFetchRows('SELECT * FROM `customoids` WHERE `customoid_passed` = 1 AN
     $user_funcs = [
         'celsius_to_fahrenheit',
         'fahrenheit_to_celsius',
+        'kelvin_to_celsius',
         'uw_to_dbm',
     ];