Start with relevant experience, not the length of the client list. Request three or better than php four case studies that sit close to your domain and your stack, and then find out who actually wrote that code. A serious vendor is happy to connect you with the tech lead. Vague answers at this stage almost always mean the demo work came from somewhere else.
The agreement deserves more attention than the sales deck. A few clauses carry most of the weight: intellectual property assignment, confidentiality, and exit terms and handover. Every artifact must transfer to you laravel vs ruby on rails payment, including documentation, pipelines and deployment scripts. Be careful with wording that keeps framework code in the vendor's hands, as that is often exactly the piece that locks you in.
Ask how they estimate. An honest estimate is accompanied by a list of assumptions, a breakdown per feature and an explicit range. A fixed price is only reasonable when the requirements are stable and documented; otherwise the supplier prices the risk in and you fund the buffer regardless. A time-and-materials model puts the risk on your side, so it needs a cap, regular demos and transparent reporting.
The delivery process matters as much as the number of developers. Establish how change requests are handled, who writes the acceptance criteria and how quality assurance works. A mature team will be able to walk you through a live build at the end of each sprint. Clear, written acceptance criteria stay your only real protection against an argument at delivery time.
Last, plan for the end of the engagement while the relationship is still good. Ask that the source repository sits under your account from the beginning, and that documentation is written as you go rather than left to the end. A partner who is comfortable with this will agree quickly; resistance at this point reveals most of what you need to know.