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What is variable scope in PHP, and why can't a function normally access a variable defined outside of it? What are default parameter values and variadic functions in PHP, and how do they add flexibility to how a function can be called? What are named arguments introduced in PHP 8, and how do they improve the readability of function calls with many parameters? How does the static keyword affect a variable declared inside a function, and how does this differ from a regular local variable? How does recursion work in PHP functions, and what are the practical considerations, such as stack depth limits, when writing a recursive function? How do function type declarations and return type declarations improve code reliability, and how does PHP handle nullable and union types in function signatures?
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How do you define and call a basic function in PHP, and what are function parameters and return values?

Beginner

Answer

You define a function in PHP using the function keyword followed by a name, a list of parameters the function accepts inside parentheses, and a body of code enclosed in curly braces, optionally using the return keyword to send a value back to whatever code called the function, letting you package up a reusable piece of logic that can be called by name whenever it is needed throughout your code.
function greet($name) {
    return "Hello, $name!";
}
echo greet('Ali');
Real-world example A pricing calculator defines a single reusable function that calculates tax on a given price, calling that same function consistently everywhere a tax calculation is needed rather than duplicating the calculation logic in multiple places.

Common follow-ups: What is the difference between a function that returns a value and one that does not?;Can a PHP function have multiple return statements?

Closures & Anonymous Functions;Type Declarations & Strict Types
All 7 Functions & Scope questions