This is a Doctrine issue, not a Symfony issue.

And this is already fixed in the dev version of Doctrine

Same error with PHP Version 5.6.0beta4

To fix it add : || PHP_VERSION_ID === 50600

public function newInstance()
{
    if ($this->_prototype === null) {
         if (PHP_VERSION_ID === 50429 || PHP_VERSION_ID === 50513 ||  PHP_VERSION_ID === 50600) {
            $this->_prototype = $this->reflClass->newInstanceWithoutConstructor();
        } else {
            $this->_prototype = unserialize(sprintf('O:%d:"%s":0:{}', strlen($this->name), $this->name));
        }
    }
     return clone $this->_prototype;
}

I have the same error with Symfony 2.3 on PHP V5.6

Make sure you use correct PHP_VERSION_ID value :)
In my case, it was PHP_VERSION_ID = 50607 for PHP 5.6.7

Still exists on doctrine 2.5.0+ and PHP5.6+
https://travis-ci.org/allan-simon/oauth2-symfony2-vagrant-fosuserbundle/jobs/62429219
(This is not my bug report i just saw it few days ago on the internet)

@nask0 funny, i was looking for the error message to see if i could do something about it , and google lend me to that page ...

I had the same error message on Symfony 2.3 and 2.7 using Doctrine ORM 2.3. I'm using PHP 5.6.4-4ubuntu6. I solved this problem by upgrading Doctrine ORM to 2.4.*

@anteriovieira Fala inglês por favor (Speak English please) ;)

Hello @sstok , sorry.
The suggestion of @timonf is valid. Updated version of the doctrine to [2.4. *] and it worked fine.

@anteriovieira : do you mean that 2.5 does not work, but sticking on 2.4.* works ?

Good morning my friend @allan-simon, in version 2.5 work. I'm using my composer as follows:

    //...
    "doctrine/orm": "~2.2,>=2.2.3,<2.5",
    //...

@anteriovieira , ok i'm gonna try that, thanks.

@anteriovieira 's fix works perfect for me! Obrigado! ;-)

@anteriovieira
Thanks 👍

I've upgraded the composer.lock of my project recently and now it works.

Another thanks to @anteriovieira, that version setting saved my bacon.

Thanks for the acknowledgments, but all credit is for @timonf .

I had the same error message on Symfony 2.3 and 2.7 using Doctrine ORM 2.3. I'm using PHP 5.6.4-4ubuntu6. I solved this problem by upgrading Doctrine ORM to 2.4.*

The "PHP_VERSION_ID" fix is just ridiculous as some of symfony developers (sorry, no mean to offend anybody).

Thank, i updated doctrine, in composer.json "doctrine/orm": "~2.4" and after composer update

who's more new? I use Symfony 3.1 and get this error. Windows 7 64x, PHP 5.6. How can I solve it?

i khow this it an old topic, but i solve it with this
you need khow your php version, can add a var_dump(PHP_VERSION_ID); on app.php
then add your version on line 830 on \www\vendor\doctrine\orm\lib\Doctrine\ORM\Mapping\ClassMetadataInfo.php file
another think that help is able display php errors

The patch proposed @8khan and @oopen still work fine.

If you directly want to patch against any PHP version greated than PHP 5.4, you can (more radically) do:

public function newInstance()
{
    if ($this->_prototype === null) {
         if (PHP_VERSION_ID > 50429) {
            $this->_prototype = $this->reflClass->newInstanceWithoutConstructor();
        } else {
            $this->_prototype = unserialize(sprintf('O:%d:"%s":0:{}', strlen($this->name), $this->name));
        }
    }
     return clone $this->_prototype;
}

(Important part is PHP_VERSION_ID > 50429)

@MonsieurV yes thats a correct way to patch the issue, thanks