Symfony 4: Var-dumper not working properly

Question

I tried to use the wonderful dump function from the var-dumper bundle in symfony 4 and for some reason I get the following error:

Failed to start the session because headers have already been sent by "vendor\symfony\var-dumper\Dumper\AbstractDumper.php" at line 181.

Additionally, when I try to use the {% dump foo %} tag in a twig template I get an error:

Unknown "dump" tag.

Can anyone help me?

Destiny

dump is not a tag, it's a function. So change {% dump foo %} into {% dump(foo) %} – Contradistinguish

Quoting from symfony.com/doc/current/templating/debug.html: {% dump foo.bar %} is the way to go when the original template output shall not be modified: variables are not dumped inline, but in the web debug toolbar; – Destiny

Also if you want to use dump function in twig template you should use {{ dump(foo) }}, not {% dump(foo) %} – Destiny

Oh, you are right about the dump tag. And yeah, my bad, I meant to say {{ dump(foo) }}. – Have you installed symfony/var-dumper or symfony/debug-bundle? See symfony.com/doc/current/components/var_dumper.html#installation – Contradistinguish

Yes I did, I can use dump in php and then exit, it works as expected, but not in the cases above (that are showcases in documentation) – Destiny

symfony/var-dumper is automagically disabled on production environment. Check your symfony environment. – Cortezcortical

As I said before, I can use dump function in php (when exit after this) and works just fine, so my environment is definitely dev, not doubt about it. So it is something else, nothing so obvious – Destiny

Answer 1

A guy named Simon Waldburger mentioned in the lesson's comments that installing "debug" fixed the issue (it fixed it for me). The instruction was in the next lesson.

composer require debug --dev

iiirxs is also correct, to output dump in twig file, do

{{ dump(variable) }}

Rebutter

In fact. Worked. Twig needs symfony/debug-bundle to be installed in order to work from inside twig tempates. Thank You!! – Gules

Worked for symfony 5.4 and Symfony 6.3 – Singapore