The order of precedence of an operator determines which operator processes first in an expression. For instance, the multiplication and division process before addition and subtraction. You can see a simplified table at http://www.zend.com/manual/ language.operators.php#language.operators.precedence.
If the operators have the same precedence, they are processed in the order they appear in the expression. For example, multiplication and division process in the order in which they appear in an expression because they have the same precedence. Operators with the same precedence can occur in any order without affecting the result.
Most expressions don't have more than one operator of the same precedence level, or the order in which they process doesn't change the result. As shown in Example 4-4, when adding and subtracting the following sequence of numbers, it doesn't matter whether you add or subtract first--the result is still 1.
Example 4-4. Order of precedence
2 + 4 - 5 == 1; 4 - 5 + 2 == 1;
4 * 5 / 2 == 10; 5 / 2 * 4 == 10;
2 + 4 - 5 == 1; 4 - 5 + 2 == 1;
When using expressions that contain operators of different precedence levels, the order can change the value of the expression. You can use parentheses, ( and ), to override the precedence levels or just to make the expression easier to read. Example 4-5 shows how to change the default precedence.
Example 4-5. Changing the default precedence using parentheses
echo 2 * 3 + 4 + 1; echo 2 * (3 + 4 + 1);
This outputs:
11 16
In the second expression, the multiplication is done last because of the parentheses overriding the default order.
PHP has several levels of precedence, enough that it's difficult to keep track of them without checking a reference. Table 4-2 is a list of PHP operators sorted by order of precedence from highest to lowest. Operators with the same level number are all of the same precedence.
The Association column lists operators that are right-to-left instead of left-to-right. We'll discuss associativity next.
