Rewriting a Legacy PHP SaaS on a Modern Stack: Berlin Cost and Method for 2026

The verdict in three sentences

For a Berlin vendor of driving school software with 1,100 customers, a progressive rewrite (strangler pattern) of its 2012 PHP 5 code costs 120,000 to 220,000 EUR over 9 to 14 months. The big bang looks cheaper on paper, but it concentrates churn risk on a single cutover and overruns its budget in most cases. With technical debt eating 35 % of team time, waiting already costs more than 100,000 EUR a year.

What technical debt really costs

The vendor's CEO sees every new feature taking three times longer than in 2018. PHP 5.6 has had no security patches since 2019, hiring fails because nobody wants to maintain this code, and the host threatens to charge for extended support.

The math is simple: 35 % of a 360,000 EUR payroll means 126,000 EUR a year spent working around existing code, not counting customers lost to slowness.

Strangler pattern or big bang: the comparison

The strangler pattern places a routing layer in front of the old application, then replaces modules one by one (lesson scheduling, billing, student tracking, exams) with new services. The big bang rewrites everything in parallel and switches over at once.

With 1,100 customers at 95 EUR per month, each churn point costs about 12,500 EUR of annual revenue. A failed big bang cutover at 8 % churn means 100,000 EUR of recurring revenue lost.

Roadmap and tracking KPIs

Months 1 to 2: audit, module mapping, routing gateway and a non-regression test suite on critical journeys. Months 3 to 5: migrate the lesson scheduling module, the most used, to a TypeScript, Next.js and PostgreSQL stack, with 50 pilot driving schools. Months 6 to 10: billing, student progress tracking and the student mobile app. Months 11 to 14: secondary modules, final data migration and PHP shutdown. Monthly KPIs: share of traffic served by the new stack, error rate, 95th percentile response time, support tickets per 100 customers, monthly churn versus the trailing 12-month average.

Mini case study

Sophie runs the Berlin vendor: 1,100 driving school customers and 1,254,000 EUR of annual recurring revenue. Berlin agencies quoted 260,000 EUR; she commits to a 170,000 EUR progressive rewrite over 12 months with a nearshore team. Debt drops from 35 % to 12 % of team time, recovering 82,800 EUR of capacity each year. By limiting extra churn to 1 % (12,500 EUR) instead of the feared 8 % with a big bang, she protects about 87,000 EUR of revenue. Estimated payback: 22 months, then a net gain of over 80,000 EUR a year.

FAQ

Should feature work freeze during the rewrite?

No, that is the point of the strangler pattern. Plan about 70 % of capacity on migration and 30 % on customer requests.

Which stack should replace PHP 5 in 2026?

PHP 8.3 with Laravel or Symfony is a serious option if the team stays PHP. TypeScript with Next.js and PostgreSQL eases hiring and code sharing with mobile.

How do we avoid churn during migration?

Communicate three weeks before each cutover, migrate 5 % of volunteer customers first and keep rollback available for 30 days.

Can part of the rewrite be funded?

Germany's research allowance (Forschungszulage, 25 % of eligible R&D staff costs) can apply to genuinely new features, not plain migration. A tax advisor must validate the scope.

What does a preliminary audit cost?

Plan 8,000 to 15,000 EUR for a 2 to 3 week audit, with a per-module estimate and a migration plan.

Let's scope your project. Share your stack, customer count and modules, and we will price a progressive rewrite between 120,000 and 220,000 EUR, with a 2 to 3 week preliminary audit. Detailed quote within 48 h. WhatsApp +221 77 596 93 33.