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When Modernization Breaks Working Code: A Study of LLMs on Legacy PHP
Brief published November 3, 2026 ยท Original source published November 1, 2026
Original reporting at arxiv.org.
Automated brief. Verify important details at the original source.
What happened
A study tested five LLMs on a single-file modernization task: each model was given 100 legacy PHP files from WordPress 4.0 and asked to modernize them. The research examines whether LLM-generated rewrites actually improve on the original code or introduce regressions, focusing on a workflow that developers commonly use by pasting a file directly into a model prompt.
Why it matters
Developers routinely use LLMs as a quick path to modernize legacy codebases, but the study raises a pointed question about whether that shortcut is safe. If models break working code during modernization, teams relying on this approach without rigorous testing could ship regressions into production systems built on widely deployed platforms like WordPress.
What to watch
The paper's full results on how often each of the five models introduced functional breakage versus genuine improvements are the key output to track. Whether findings generalize beyond PHP or single-file contexts remains an open question the supplied evidence does not resolve.