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PHP has two anonymous-function styles. Standard closures:

$double = function ($n) {
    return $n * 2;
};

echo $double(5);   // 10

Closures don't see outer scope variables by default — you have to opt in with use:

$factor = 3;
$multiply = function ($n) use ($factor) {
    return $n * $factor;
};

Arrow functions (PHP 7.4+) are shorter and capture outer variables automatically:

$factor = 3;
$multiply = fn($n) => $n * $factor;   // implicit return, auto-capture

Limitations: arrow functions are single-expression only and capture by value (not reference).

Closures are first-class — pass them to array_map, array_filter, array_reduce, usort, etc.