Since 19th June 2020, this rule applies:

1. Formal namespaces are required for any new classes in Moodle. The following exceptions apply:
   1. there is no requirement to move existing non-namespaced classes to a namespace; and
   2. where an existing mechanism exists for loading a class, and that mechanism does not support the use of a namespaced class, the existing pseudo-namespaced prefix on the class name will be allowed.
2. The use of a pseudo-namespace prefix on classnames is deprecated and should only be used in the above situations.
3. The use of namespaces must conform to the following rules:
   1. Classes belonging to a namespace must be created in a classes directory inside a plugin (e.g. mod/forum/classes), or for core code, in lib/classes or subsystemdir/classes.
   2. The classname and filename for all namespaced classes must conform to the automatic class loading rules. The use of formal PHP namespaces in all new code is required.

Valid examples:

namespace mod_forum;

class example {

}

namespace mod_forum\external;

class example {

}

namespace core_user;

class example {

}

No longer valid:

class mod_forum_example {

}

class mod_forum_external_invalid {

}

class core_user_example {

}