::class is available since PHP 5.5 (5.5 is a minimal requirement by Phorge): https://www.php.net/manual/en/language.oop5.basic.php#language.oop5.basic.class.class
It makes finding code using IDEs easier; see discussion in D25500.

Thus replace all string return values with returning the ::class constant instead, with one exception: 'PhabricatorSettingApplication' in PhabricatorUserPreferencesSearchEngine.php does not exist and makes arc lint fail so this string remained unchanged.

Also note that two occurrences were wrapped in pht() for reasons I do not know.

List of functions whose return value get updated in this code change:

• getApplicationClassName()
• getAdapterApplicationClass()
• getDatasourceApplicationClass()
• getEditorApplicationClass()
• getEngineApplicationClass()
• getPHIDTypeApplicationClass()
• getQueryApplicationClass()

cf. T15158

Too broad - click around, basically.