The PHP interpreter, powering over 70% of websites 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 silently lead to incorrect results. We observe that the introduction of Just-In-Time (JIT) compila- tion mode in PHP presents an opportunity for differential testing, as it provides an alternative implementation of the same language specification. To shed light on this, we propose ZendDiff, an automatic differential testing framework that efficiently detects logic bugs in the PHP interpreter by comparing JIT and non- JIT execution results. Our differential testing incorporates three key techniques: program state probing for fine-grained execution state comparison, JIT-aware program mutation for sufficiently exercising JIT functionality, and dual verification to handle non- deterministic behaviors in PHP programs. Our experimental results demonstrate that ZendDiff outperforms 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, ZendDiff has identified 51 previously unknown logic bugs in the PHP interpreter, with 37 already fixed and 3 confirmed by PHP developers. ZendDiff has been acknowledged by the PHP developers, and offers a practical tool for automatically discovering logic bugs in the PHP interpreter.