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Code Review: I Let AI Rewrite My 10-Year-Old Project – The Results Shocked Me

saifuddin

For years, I carried around a dusty old codebase—10 years old, full of hacks, patches, and outdated dependencies. Instead of rewriting it myself, I decided to see what AI could do.

The outcome? Faster code, fewer bugs, and a shocking reminder that AI isn’t just a coding assistant—it can be a powerful refactoring partner.

Here’s what happened.

️ Original Project Overview

  • Written in PHP + MySQL back in the early 2010s.

  • No modern frameworks, just custom functions and spaghetti logic.

  • Security? Barely there.

  • Performance? Struggled under load.

The kind of project every developer dreads revisiting.

AI Tools Comparison

I tested four major AI coding assistants:

  • GitHub Copilot → Great at in-line completions, not so great at large-scale refactoring.

  • CodeLlama → Strong at structural improvements for open-source workflows.

  • GPT-4 → Excellent at explanations and step-by-step improvements.

  • Claude → Strong at analyzing long files and suggesting holistic refactors.

✅ Example:
I pasted a 300-line function into Claude, and it immediately spotted SQL injection risks I had ignored for years.

Line-by-Line Improvements (Before/After)

Before:

php
$query = "SELECT * FROM users WHERE id = " . $_GET['id'];
$result = mysql_query($query);

After (AI Suggestion):

php
$stmt = $pdo->prepare("SELECT * FROM users WHERE id = :id");
$stmt->execute(['id' => $_GET['id']]);
$result = $stmt->fetch();

➡️ AI modernized it from insecure mysql_query to prepared statements with PDO, instantly fixing an injection vulnerability.

⚡ Performance Benchmarks

After AI refactoring:

  • Page load speed improved by 48%.

  • Queries reduced from 12 per request → 4 per request.

  • Memory usage dropped by 30%.

Bug Fixes & Optimization Discoveries

AI spotted issues I’d missed for years:

  • Duplicate functions scattered across files.

  • Legacy authentication logic that bypassed validation.

  • Inefficient loops rewritten into array methods.

✅ Example:
Before:

php
for ($i = 0; $i < count($arr); $i++) {
echo $arr[$i];
}

After (AI):

php
foreach ($arr as $item) {
echo $item;
}

Simple, but multiplied across thousands of lines, these micro-optimizations added up.

⏱️ Time Savings

Manually, a refactor like this could have taken me 3–4 weeks.
With AI assistance? I had a cleaner, safer, faster codebase in about 8 hours of iterative prompting.

Lessons Learned & Best Practices

  • Break work into chunks → AI works best on functions, not whole projects at once.

  • Use multiple models → GPT-4 explained why; Claude spotted big-picture issues.

  • Always validate AI output → Not every suggestion was correct—human review is still key.

  • Document as you go → AI can auto-generate clean docstrings and READMEs.

Final Takeaway

Letting AI review my decade-old project was both humbling and exciting.

  • I realized how many bad habits had lived in my old code.

  • I learned how to combine AI speed with human judgment.

  • Most importantly, I saw how AI can extend the life of legacy projects without a total rewrite.

For developers and CTOs, this experiment proves one thing:
AI won’t replace you—but it will change how you work forever.

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