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How do you define and call a basic function in PHP, and what are function parameters and return values? 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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What is variable scope in PHP, and why can't a function normally access a variable defined outside of it?

Beginner

Answer

Variable scope determines where in your code a particular variable can be accessed, and in PHP, a variable defined inside a function is local to that function by default and cannot be accessed from outside it, while a variable defined outside any function exists in the global scope but is also not automatically accessible from within a function unless explicitly imported using the global keyword, a design intended to prevent functions from accidentally interfering with unrelated parts of a program.
$total = 100;
function showTotal() {
    echo $total ?? 'not accessible here';
}
showTotal();
Real-world example A developer troubleshooting an undefined variable warning realizes their function was trying to directly use a variable defined outside of it, and fixes the issue by properly passing that value into the function as a parameter instead.

Common follow-ups: Why does PHP intentionally isolate function scope from the global scope by default?;What is the difference between using the global keyword and simply passing a variable as a parameter?

Basics & Types;Closures & Anonymous Functions
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