The verdict in three sentences
For a 12-year-old PHP 5 application, a full rewrite costs between EUR 120,000 and 300,000 excl. VAT and concentrates all the risk on a single cutover after 12 to 18 months. Progressive modernisation module by module (the strangler pattern), at EUR 15,000 to 40,000 per module, delivers value every two to three months and limits regression risk. When current maintenance already costs EUR 60,000 a year, doing nothing is the most expensive option in the medium term.
What the status quo really costs
PHP 5 has received no security patch since 2018. For a health insurer processing medical data, exposure is real: known vulnerabilities, hosts refusing old versions, developers increasingly scarce and expensive.
| Annual cost item | Current PHP 5 app | After modernisation | Difference |
|---|---|---|---|
| Corrective maintenance | EUR 35,000 | EUR 12,000 | -EUR 23,000 |
| Specific hosting (old stack) | EUR 9,000 | EUR 4,000 | -EUR 5,000 |
| Security workarounds (WAF, audits) | EUR 8,000 | EUR 3,000 | -EUR 5,000 |
| Scarce developer premium | EUR 8,000 | EUR 0 | -EUR 8,000 |
| Total maintenance | EUR 60,000 | EUR 19,000 | -EUR 41,000 |
| Average lead time for a simple change | 15 days | 3 to 5 days | 3 times faster |
These are 2026 ballpark figures for a business application of 80 to 150 screens used by 200 to 400 staff.
Full rewrite or progressive modernisation: the comparison
A rewrite looks attractive on paper: new architecture, clean code, a single migration. In practice it freezes changes for a year and forces the team to reproduce 12 years of often undocumented business rules.
| Criterion | Full rewrite | Progressive modernisation by module |
|---|---|---|
| Total budget | EUR 120,000 to 300,000 | EUR 15,000 to 40,000 per module, EUR 90,000 to 220,000 total |
| Duration | 12 to 18 months | 6 to 18 months, spread out |
| First production release | End of project | 2 to 3 months |
| Regression risk | High, single cutover | Limited, module by module |
| Feature freeze | 9 to 12 months | None |
| Old and new coexist | No | Yes, via proxy and API |
| Best fit | Small or functionally obsolete app | Critical app with rich business rules |
Progressive modernisation puts a proxy in front of the old application: each rewritten module (members, contributions, claims, reporting) takes over from the old one until the legacy system is switched off.
A typical progressive modernisation plan
- Month 1: audit and mapping, EUR 8,000 to 15,000, identifying modules and database dependencies.
- Months 2 to 3: technical foundation (PHP 8.3 and Symfony, or Node.js), authentication, proxy, automated tests on the existing app.
- Months 3 to 12: one module every 2 to 3 months, starting with the one that costs most to maintain.
- Months 12 to 18: migration of remaining data, legacy shutdown, documentation.
- Continuously: regression tests on sensitive calculations (contributions, reimbursements), with a set of 200 to 500 real anonymised cases.
Mini case study
Katrin, CIO of a health insurer in Berlin (300 staff), maintains a 12-year-old PHP 5 application costing EUR 60,000 a year. A first quote for a full rewrite comes in at EUR 240,000 over 16 months, with a freeze on regulatory changes she cannot accept.
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She chooses progressive modernisation: a EUR 12,000 audit, then 5 modules priced between EUR 22,000 and 38,000, i.e. EUR 150,000 in total over 15 months. As soon as the "contributions" module ships in month 4, maintenance drops by about EUR 15,000 a year. Eventually savings reach EUR 41,000 a year. Net cost compared with the rewrite: EUR 90,000 less, and a payback of about 3.5 years on maintenance alone, before counting changes delivered three times faster.
FAQ
Can we simply upgrade from PHP 5 to PHP 8 without modernising?
Sometimes, if the code is clean. For a 12-year-old app, a version upgrade often costs EUR 20,000 to 50,000 and leaves functional debt untouched.
How long does progressive modernisation of a business app take?
Between 6 and 18 months depending on the number of modules. Each module typically takes 6 to 12 weeks including acceptance testing.
How do we avoid regressions on contribution calculations?
By first building a test suite from 200 to 500 real anonymised files, run against both old and new code. This represents 10 to 15% of the budget.
Is a full rewrite ever the right choice?
Yes, when the app is small (under 30 screens), business processes have fundamentally changed or the source code is unusable.
Should we keep the same vendor for maintenance?
It is not mandatory, but require a Git repository in your name and per-module documentation to stay free. Support for the new app then costs 15 to 20% of the modernisation budget per year.
Let's scope your project. Send us the PHP version, the number of screens and your current maintenance budget: we will price an audit and a module-by-module modernisation plan. Detailed quote within 48 h. WhatsApp +221 77 596 93 33.
Mohamed Bah
Fondateur, Kolonell
Passionate about digital and entrepreneurship in Africa, Mohamed has been helping Sénégalese businesses with their digital transformation since 2020. Founder of Kolonell, he believes every SME deserves a professional and accessible online présence.
