When choosing a Custom PHP Development Service provider, it's important to consider several factors. Firstly, look for a provider with a strong track record of delivering high-quality solutions that meet their clients' needs. Secondly, consider the provider's expertise and experience with PHP development. Thirdly, check that the provider has a robust quality assurance process in place to ensure that their solutions are bug-free and meet your requirements. Finally, consider the provider's pricing and whether it fits within your budget.