The $convert aggregation expression's base parameter (added in MongoDB 8.3) accepts any aggregation expression server-side (parsed via parseOperand() in expression.cpp).

The PHP builder should support both:

• A closed enum of allowed literal values: 2 (binary), 8 (octal), 10 (decimal), 16 (hexadecimal)
• Any expression that resolves to an integer (e.g. a field path, variable, or computed expression)

This mirrors the pattern used for other builder parameters that accept either a typed literal or a generic expression.

Reference:

• MQL spec PR: https://github.com/mongodb/mql-specifications/pull/41
• Server source: src/mongo/db/pipeline/expression.cpp