PHP Decision-Making - Order of precedence

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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.

Table 4-2. List of PHP operators

Operator	Description	Operands	Association	Level
NEW	Create new object	Constructor call	Right to left	1
.	Property access (dot notation)	Objects		2
[ ]	Array index	Array, integer, or string		2
()	Function call	Function or argument		2
!	Logical NOT	Unary	Right to left	3
~	Bitwise NOT	Unary	Right to left	3
++, -	Increment and decrement operators	1value	Right to left	3
+,	Unary plus, negation	Number	Right to left	3
(int)	Cast operators	Unary	Right to left	3
(double)	Cast operators	Unary	Right to left	3
(string)	Cast operators	Unary	Right to left	3
(array)	Cast operators	Unary	Right to left	3
(object)	Cast operators	Unary	Right to left	3
@	Inhibit errors	Unary	Right to left	3
*, /, %	Multiplication, division	Numbers		4
+,	Addition, subtraction	Numbers		5
.	Concatenation	Strings		5
<<, >>	Bitwise shift left, bitwise shift right	Binary		6
<, <=, >, >=	Comparison operators	Numbers, strings		7
==, !=	Equality, inequality	Any		8
===, !==	Identity, nonidentity	Any		8
&	Bitwise AND	Binary		9
^	Bitwise NOR	Binary		10
|	Bitwise OR	Binary		11
&&	Logical AND	Boolean		12
||	Logical OR	Boolean		13
? :	Conditional	Boolean	Right to left	14
=	Assignment	1value=any	Right to left	15
AND	Logical AND	Boolean		16
OR	Logical OR	Boolean		17
XOR	Logical XOR	Boolean		18

Next: Associativity >>
 

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