On the Exakat blog there's a post that covers assertions in PHP, a built-in tool the language provides to help perform simple value based evaluation against certain criteria.

PHP has a clever native debugging tool : the PHP assertions. In a nutshell, assertions are a function call to assert(), that triggers an error when a condition is not satisfied.

[...] Unlike debugging conditions, the assert() syntax has to be read in a positive way. When the condition, aka as the first argument, is satisfied, all is fine, and process goes on; when the condition is not satisfied, then an error is triggered: the message of the error is the second argument.

The post then gets into more detail about the assertions, noting that the result has to be positive and that they can be somewhat configured (basically turned on/off and the error level can be adjusted). It also covers some examples of things to test with assertions, how they should be treated as debugging and to avoid using them on resources outside the code (like database connections).