<?php

namespace DJBlaster\BlasterBundle\Form\Type;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolverInterface;
use Symfony\Component\Form\Extension\Core\Type\SubmitType;

class UserType extends AbstractType {
    public function buildForm(FormBuilderInterface $builder, array $options) {
        $builder -> add('name') -> add('email') -> add('save', SubmitType::class);

    }

    public function getBlockPrefix() {
        return 'user';
    }

    public function setDefaultOptions(OptionsResolverInterface $resolver) {
        $resolver -> setDefaults(array('data_class' => 'DJBlaster\BlasterBundle\Entity\User', 'cascade_validation' => true, ));
    }
}
<?php

namespace DJBlaster\BlasterBundle\Form\Type;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolverInterface;

class UserType extends AbstractType {
    public function buildForm(FormBuilderInterface $builder, array $options) {
        $builder -> add('name') -> add('email') -> add('save', 'submit');

    }

    public function getBlockPrefix() {
        return 'user';
    }

    public function setDefaultOptions(OptionsResolverInterface $resolver) {
        $resolver -> setDefaults(array('data_class' => 'DJBlaster\BlasterBundle\Entity\User', 'cascade_validation' => true, ));
    }
}
--- 
+++ 
@@ -5,10 +5,11 @@
 use Symfony\Component\Form\AbstractType;
 use Symfony\Component\Form\FormBuilderInterface;
 use Symfony\Component\OptionsResolver\OptionsResolverInterface;
+use Symfony\Component\Form\Extension\Core\Type\SubmitType;
 
 class UserType extends AbstractType {
     public function buildForm(FormBuilderInterface $builder, array $options) {
-        $builder -> add('name') -> add('email') -> add('save', 'submit');
+        $builder -> add('name') -> add('email') -> add('save', SubmitType::class);
 
     }
 
Replace string submit with SubmitType.
