Why is this an issue?

A long parameter list can indicate that a new structure should be created to wrap the numerous parameters or that the function is doing too many things.

Noncompliant code example

With a maximum number of 4 parameters:

function doSomething($param1, $param2, $param3, $param4, $param5) {
...
}

Compliant solution

function doSomething($param1, $param2, $param3, $param4) {
...
}