The Thrift IDL (https://thrift.apache.org/docs/idl) specifies php_namespace as being followed by a literal:
( 'php_namespace' Literal ) |
Literals are required to have quotes (either double quotes or single quotes). The Thrift tutorial (http://wiki.apache.org/thrift/Tutorial) has php_namespace followed by an identifier:
The spec and tutorial should agree with each other (either both Literal and Identifier should be allowed after php_namespace, or the tutorial should use quotes when specifying the php_namespace, and the compiler should verify that the php_namespace is a Literal in quotes).