When queries are executed in PHP, the rows are mapped to an object and columns are represented by object properties. This imposes certain limitations to the name of column within a database.
For instance, it is technically possible to name a column something-like-this, Or even this, but having hyphens or spaces is not sensible in an object property and will cause confusion.
Columns, as with object properties, should be named to describe what they represent.
As with all variable naming, special notation should not be used to identify different types of variables. This also applies to database column names; a foreign key does not need to be named in a way to identify that it is a foreign key.
Common bad examples include column names such as fk_user, but a better name would be userId, assuming the foreign key matches a user's id.
Given example tables called customer and user, both of which have an id column, if a customer column userId refers to the user table's id column, a foreign key can be named customer__user__id, or a similar naming convention should be used consistently.