Originally published on php2rust.dev — part of the PHP to Rust course. View this lesson on the original site.

When you need a variable that can change, use mut in Rust. In PHP, variables are mutable by default, so no extra keyword is needed.

Rust

fn main() {
    let mut x = 5;
    println!("The value of x is: {}", x);
    x = 6;  // Allowed because x is mutable
    println!("The value of x is: {}", x);

    let mut counter = 0;
    counter += 1;
    counter += 1;
    println!("Counter: {}", counter);  // 2
}

mut makes it clear where values can be modified. The compiler still enforces safety (e.g. no data races).

PHP

<?php
$x = 5;
echo "The value of x is: $x\n";
$x = 6;  // Allowed, PHP variables are mutable by default
echo "The value of x is: $x\n";

$counter = 0;
$counter++;
$counter++;
echo "Counter: $counter\n";  // 2

In PHP, mutability is the default. There is no keyword to mark a variable as mutable.

Comparison

Rust: Use let mut for mutable variables; mutation is explicit and compiler-checked.
PHP: Variables are mutable by default; no keyword needed.

Using mut in Rust makes mutation visible in the code and helps the compiler prevent unintended sharing or races.

References

What's next

Next: Shadowing.

From PHP to Rust — Rust Syntax and Functions