ZendDiff: Differential Testing of PHP Interpreter

The PHP interpreter, powering over 70% of web-sites on the internet, plays a crucial role in web development. Existing approaches to finding bugs in PHP primarily focus on detecting explicit security issues through crashes or sanitizer-based oracles, but fail to identify logic bugs that can silently lead to incorrect results. We observe that the introduction of Just-In-Time (JIT) compilation mode in PHP presents an opportunity for differential testing, as it provides an alternative implementation of the same language specification. We propose, ZendDiff, an automatic differential testing framework that effectively detects logic bugs in the PHP interpreter by comparing JIT and non-JIT execution results. Our differential testing incorporates three techniques: program state probing for fine-grained execution state comparison, JIT-aware program mutation to sufficiently exercise JIT functionality, and dual verification to handle non-deterministic behaviors in PHP programs. Our experimental results demonstrate that ZendDiffoutperforms the official test suite used in PHP's continuous integration, achieving higher code coverage and executing more Zend opcodes. Through ablation studies, we validate the effectiveness of these techniques. To date, ZendDiffhas identified 51 previously unknown logic bugs in the PHP interpreter, with 37 already fixed and 3 confirmed by the PHP maintainers. ZendDiffhas been acknowledged by the PHP community and offers a practical tool for automatically discovering logic bugs in the PHP interpreter.