What does ~ tilde symbol mean in the YML configuration file in Symfony 2

What does ~ tilde symbol mean in the YML configuration file in Symfony 2

In Yaml, ~ means null.

The actual meaning in Symfony 2 depends on what you are configuring. Most likely it will mean "use defaults" or it will be used where it is needed to put a configuration key but there is nothing to actually configure for that key (as in the NotBlank example).

Reference:

http://stackoverflow.com/questions/8667766/what-does-mean-in-the-yml-configuration-file-in-symfony-2