Add validation function for phone numbers

src/MD_STD_IN.php

@@ -212,6 +212,27 @@ final class MD_STD_IN {
 
     }
 
+    /**
+     * Sanitizes and validates a phone number. An empty string passes.
+     *
+     * @param mixed $input Input string.
+     *
+     * @return string
+     */
+    public static function validate_phone_number(mixed $input):string {
+
+        if ($input === "") {
+            return "";
+        }
+
+        if (!preg_match("#^[()0-9/ +-]+$#", $input)) {
+            throw new MDgenericInvalidInputsException("Invalid phone number entered.");
+        }
+
+        return $input;
+
+    }
+
     /**
      * Sanitizes a string to a float.
      *