symfony PHP5 framework Chapter 5 - Configuring Symfony

Using Constants in YAML Configuration Files

Some configuration settings rely on the value of other settings. To avoid setting the same value twice, symfony supports constants in YAML files. On encountering a setting name (one that can be accessed by sfConfig::get()) in capital letters enclosed in % signs, the configuration handlers replace them with their current value. See Listing 5-20 for an example.

Listing 5-20 - Using Constants in YAML Files, Example from autoload.yml

autoload:
  symfony:
    name:           symfony
    path:           %SF_SYMFONY_LIB_DIR%
    recursive:      on
    exclude:        [vendor]

The path parameter will take the value returned by sfConfig::get('sf_symfony_lib_dir'). If you want one configuration file to rely on another, you need to make sure that the file you rely on is already parsed (look in the symfony source to find out the order in which the configuration files are parsed). app.yml is one of the last files parsed, so you may rely on others in it.