Parenthesis
Parenthesis are the ( and ). They must be balanced: each opening parenthesis must have a closing parenthesis, or a PHP syntax error is generated.
Parenthesis are used to make explicit precedence between operations. Parenthesis group all underlying operations as one call: there can be only one expression inside parenthesis.
Parenthesis are an important token for the parser: they are needed in many situations, such as method definition, method call, for(), foreach(), switch(), match(), etc. Once the syntax has been recognized, such parenthesis are not used anymore.
Parenthesis are idempotent: there is no use in setting parenthesis directly inside another parenthesis.
Sometimes, parenthesis are used for make language structure look like a functioncall. This is useless to the PHP engine, and even, prevent some features.
<?php
// parenthesis in signatures
function trumpet($parameter) {}
// useless parenthesis
include('/path/to/file.php');
// parenthesis
echo 'a', 'b'; // best practice: no parenthesis
echo ('a' . 'b'); // parenthesis force the concatenation
echo ('a'), 'b'; // weird, right?
// precedence changing with parenthesis
echo 2 + 3 * 4; // 14
echo (2 + 3) * 4; // 20
echo 2 + (3 * 4); // 14
?>