PHPDoc: Replace non-existing return type phid with string
Summary:
`phid` is not a valid type. See https://docs.phpdoc.org/guide/references/phpdoc/types.html and https://docs.phpdoc.org/guide/references/phpdoc/tags/return.html.
Thus return `string` and get less errors reported by PHPStan.
Test Plan: Read docs; run static code analysis.
Reviewers: O1 Blessed Committers, valerio.bozzolan
Reviewed By: O1 Blessed Committers, valerio.bozzolan
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Differential Revision: https://we.phorge.it/D25944