The order in which operators are evaluated is determined by the operator precedence (see also the Remarks section).
$a = 2 * 3 + 4;
$a gets a value of 10 because
2 * 3 is evaluated first (multiplication has a higher precedence than addition) yielding a sub-result of
6 + 4, which equals to 10.
The precedence can be altered using parentheses: in
$a = 2 * (3 + 4);
$a gets a value of 14 because
(3 + 4) is evaluated first.