php operator precedence order

Operator precedenceSpecifies how "tightly" two expressions are bound. For example, the expression 1 + 5 * 3 evaluates to 16 instead of 18 because the multiplication sign ("*") has higher precedence than the plus sign ("+"). Parentheses can be used to force a priority change if necessary. For example: (1 + 5) * 3 has the value 18.

If operators have the same precedence, their combination direction determines whether evaluation should be from right to left or from left to right - see the example below.

The following table lists the operators from high to low precedence. Operators in the same line have the same precedence, and the direction in which they are combined determines the order of evaluation.

##None== != === !== <>Comparison operatorleft&Bit operators and referencesleft^Bit operatorsleft|Bitwise operatorsleft&Logical operatorsleft||logical operatorleft? :ternary operatorright= += -= *= /= .= %= &= |= ^= <<= > >= =>##leftleftleftleft

Combining direction	Operator	Additional information
无	clone new	clone and new
left	[	array( )
right	++ — ~ (int) (float) (string) (array) (object) (bool) @	Type and increment/decrement
None	instanceof	Type
right	!	Logical operator
left	* / %	Arithmetic operators
left	+ – .	Arithmetic operators and String operators
left	<< >>	Bitwise operator
		Assignment operator
and	Logical operator
xor	logical operator
or	Logical operators
,	are used in many places

For operators with the same precedence, the left associative direction means that the evaluation will be from left to right, and the right associative direction means the opposite. It is possible that an operator with the same precedence without associative direction cannot be combined with itself. For example, in PHP 1 < 2 > 1 is an illegal statement, but 1 <= 1 == 1 is not. Because the priority of the T_IS_EQUAL

operator
is lower than that of the T_IS_SMALLER_OR_EQUAL operator. Example Combined with the direction, the code is as follows

<?php 
$a = 3 * 3 % 5; // (3 * 3) % 5 = 4 
$a = true ? 0 : true ? 1 : 2; // (true ? 0 : true) ? 1 : 2 = 2 
$a = 1; 
$b = 2; 
$a = $b += 3; // $a = ($b += 3) -> $a = 5, $b = 5 
// mixing ++ and + produces undefined behavior 
$a = 1; 
echo ++$a + $a++; // may print 4 or 5 
?>

PS: Using parentheses, even when not strictly needed, can usually enhance the readability of the code.

Although = has lower precedence than most other operators, PHP still allows expressions like the following: if (!$a = foo()), in this case the return value of foo() is assigned to $a.

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