add substring function

Author: avrahamappel

src/Functions/Substring.php

@@ -0,0 +1,15 @@
+<?php
+
+namespace Appel\SqliteFunctions\Functions;
+
+use Closure;
+
+class Substring extends AbstractFunction
+{
+    public function mysql(): Closure
+    {
+        return function ($string, $start, $length = null) {
+            return substr($string, $start, $length);
+        };
+    }
+}