<?php

namespace TFox\MpdfPortBundle\DependencyInjection;

use Symfony\Component\Config\Definition\Builder\TreeBuilder;
use Symfony\Component\Config\Definition\ConfigurationInterface;

/**
 * This is the class that validates and merges configuration from your app/config files
 *
 * To learn more see {@link http://symfony.com/doc/current/cookbook/bundles/extension.html#cookbook-bundles-extension-config-class}
 */
class Configuration implements ConfigurationInterface
{
    /**
     * {@inheritDoc}
     */
    public function getConfigTreeBuilder()
    {
        $treeBuilder = new TreeBuilder('t_fox_mpdf_port');
        $rootNode = method_exists(TreeBuilder::class, 'getRootNode')
            ? $treeBuilder->getRootNode()
            : $treeBuilder->root('t_fox_mpdf_port');

        // Here you should define the parameters that are allowed to
        // configure your bundle. See the documentation linked above for
        // more information on that topic.

        return $treeBuilder;
    }
}

<?php

namespace TFox\MpdfPortBundle\DependencyInjection;

use Symfony\Component\Config\Definition\Builder\TreeBuilder;
use Symfony\Component\Config\Definition\ConfigurationInterface;

/**
 * This is the class that validates and merges configuration from your app/config files
 *
 * To learn more see {@link http://symfony.com/doc/current/cookbook/bundles/extension.html#cookbook-bundles-extension-config-class}
 */
class Configuration implements ConfigurationInterface
{
    /**
     * {@inheritDoc}
     */
    public function getConfigTreeBuilder()
    {
        $treeBuilder = new TreeBuilder();
        $rootNode = $treeBuilder->root('t_fox_mpdf_port');

        // Here you should define the parameters that are allowed to
        // configure your bundle. See the documentation linked above for
        // more information on that topic.

        return $treeBuilder;
    }
}

--- 
+++ 
@@ -17,8 +17,10 @@
      */
     public function getConfigTreeBuilder()
     {
-        $treeBuilder = new TreeBuilder();
-        $rootNode = $treeBuilder->root('t_fox_mpdf_port');
+        $treeBuilder = new TreeBuilder('t_fox_mpdf_port');
+        $rootNode = method_exists(TreeBuilder::class, 'getRootNode')
+            ? $treeBuilder->getRootNode()
+            : $treeBuilder->root('t_fox_mpdf_port');
 
         // Here you should define the parameters that are allowed to
         // configure your bundle. See the documentation linked above for
Fix Symfony 4.2 deprecation warning
