Doctrine doesn't allow to to create own SQL function for DQL if that function is already defined as "built-in". An example could be custom DATE_ADD implementation.
Method FunctionDeclaration() in Doctrine\ORM\Query\Parser gives higher priority to built-in SQL functions, even if they are not
usable for a specific situation, and registering of own datetime function doesn't help. This issue makes it impossible to use some advanced Doctrine extensions,
for example https://github.com/beberlei/DoctrineExtensions that provide fuller implementations.
Considering the fact that someone may want to use ready components provided by the community, and being new to Doctrine can't figure out the way to hack
or workaround this, the issue is a major one.