[php-src] PR #23110: Optimize serialize

Pull Request: https://github.com/php/php-src/pull/23110 Author: kamil-tekiela NOTE: I did not write this PR. This is part of my experiments with Claude. This PR was created entirely by Claude Sonnet 5 and tested by me. Benchmarking shows that there is real improvement, although the results vary depending on the size and contents of the payload. There doesn't seem to be any performance degradation in a normal case. Code comments have been committed separately so that they can be discarded. Please treat this as an experiment. If you don't feel comfortable reviewing or merging it, please don't.