Optimizing Closures in PHP 8.6

In PHP 8.6, I propose two key optimizations for closures, including arrow functions, which may introduce some backward compatibility (BC) breaks but promise significant performance gains. The first optimization involves converting non-static closures into static ones when they don't utilize $this, eliminating unnecessary reference cycles that previously required PHP's cycle collector to resolve. This change reduces memory usage and enhances performance by avoiding cycles that don't need to exist.

The second optimization focuses on caching stateless closures—those that are static and don't capture or declare variables—between uses. This prevents the creation of redundant closure instances, as demonstrated by a benchmark that showed an 80% performance improvement in a synthetic test. In practical applications like Laravel, this optimization reduced closure instantiations by 65%, leading to a 3% performance boost.

While these changes introduce some BC considerations, such as ReflectionFunction::getClosureThis() returning NULL for inferred static closures, these are generally predictable and align with expected behaviors. Additionally, two stateless closures from the same lexical location will now be identical, which is a shift from previous behavior.

To maintain backward compatibility, Closure::bind() and Closure::bindTo() will allow binding objects to inferred static closures, discarding them only when explicitly static. This ensures that closures inferred as static due to unrelated changes remain compatible.

The community vote on these optimizations was overwhelmingly positive, with 24 votes in favor and none against, indicating strong support for these enhancements in PHP 8.6.