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Operator Precedence

The precedence of an operator specifies how "tightly" it binds two expressions together. For example, in the expression 1 + 5 * 3, the answer is 16 and not 18 because the multiplication ("*") operator has a higher precedence than the addition ("+") operator. Parentheses may be used to force precedence, if necessary. For instance: (1 + 5) * 3 evaluates to 18.

When operators have equal precedence their associativity decides how the operators are grouped. For example "-" is left-associative, so 1 - 2 - 3 is grouped as (1 - 2) - 3 and evaluates to -4. "=" on the other hand is right-associative, so $a = $b = $c is grouped as $a = ($b = $c).

Operators of equal precedence that are non-associative cannot be used next to each other, for example 1 < 2 > 1 is illegal in PHP. The expression 1 <= 1 == 1 on the other hand is legal, because the == operator has a lower precedence than the <= operator.

Associativity is only meaningful for binary (and ternary) operators. Unary operators are either prefix or postfix so this notion is not applicable. For example !!$a can only be grouped as !(!$a).

Use of parentheses, even when not strictly necessary, can often increase readability of the code by making grouping explicit rather than relying on the implicit operator precedence and associativity.

The following table lists the operators in order of precedence, with the highest-precedence ones at the top. Operators on the same line have equal precedence, in which case associativity decides grouping.
Operator Precedence
Associativity Operators Additional Information
(n/a) clone new clone and new
right ** arithmetic
(n/a) + - ++ -- ~ (int) (float) (string) (array) (object) (bool) @ arithmetic (unary + and -), increment/decrement, bitwise, type casting and error control
left instanceof type
(n/a) ! logical
left * / % arithmetic
left + - . arithmetic (binary + and -), array and string (. prior to PHP 8.0.0)
left << >> bitwise
left . string (as of PHP 8.0.0)
left |> pipe
non-associative < <= > >= comparison
non-associative == != === !== <> <=> comparison
left & bitwise and references
left ^ bitwise
left | bitwise
left && logical
left || logical
right ?? null coalescing
non-associative ? : ternary (left-associative prior to PHP 8.0.0)
right = += -= *= **= /= .= %= &= |= ^= <<= >>= ??= assignment
(n/a) yield from yield from
(n/a) yield yield
(n/a) print print
left and logical
left xor logical
left or logical
(n/a) throw throw

Example #1 Associativity

<?php
$a = 3 * 3 % 5; // (3 * 3) % 5 = 4
var_dump($a);

$a = 1;
$b = 2;
$a = $b += 3; // $a = ($b += 3) -> $a = 5, $b = 5
var_dump($a, $b);
?>

The ternary operator specifically requires the use of parenthesis to disambiguate precedence.

Example #2 Explicit Precedence

<?php
$a = true ? 0 : (true ? 1 : 2);
var_dump($a);

// this is not allowed since PHP 8
// $a = true ? 0 : true ? 1 : 2;
?>

Operator precedence and associativity only determine how expressions are grouped, they do not specify an order of evaluation. PHP does not (in the general case) specify in which order an expression is evaluated and code that assumes a specific order of evaluation should be avoided, because the behavior can change between versions of PHP or depending on the surrounding code.

Example #3 Undefined order of evaluation

<?php
$a = 1;
echo $a + $a++; // may print either 2 or 3

$i = 1;
$array[$i] = $i++; // may set either index 1 or 2
?>

Example #4 +, - and . precedence

<?php
$x = 4;
// this line might result in unexpected output:
echo "x minus one equals " . $x-1 . ", or so I hope\n";

// the desired precedence can be enforced by using parentheses:
echo "x minus one equals " . ($x-1) . ", or so I hope\n";

// this is not allowed, and throws a TypeError:
echo (("x minus one equals " . $x) - 1) . ", or so I hope\n";
?>

The above example will output:

-1, or so I hope
-1, or so I hope
Fatal error: Uncaught TypeError: Unsupported operand types: string - int

Example #5 Prior to PHP 8, +, - and . had the same precedence

<?php
$x = 4;
// this line might result in unexpected output:
echo "x minus one equals " . $x-1 . ", or so I hope\n";

// because it is evaluated like this line (prior to PHP 8.0.0):
echo (("x minus one equals " . $x) - 1) . ", or so I hope\n";

// the desired precedence can be enforced by using parentheses:
echo "x minus one equals " . ($x-1) . ", or so I hope\n";
?>

The above example will output:

-1, or so I hope
-1, or so I hope
x minus one equals 3, or so I hope

Note:

Although = has a lower precedence than most other operators, PHP will still allow expressions similar to the following: if (!$a = foo()), in which case the return value of foo() is put into $a.

Changelog

Version Description
8.0.0 String concatenation (.) now has a lower precedence than arithmetic addition/subtraction (+ and -) and bitwise shift left/right (<< and >>); previously it had the same precedence as + and - and a higher precedence than << and >>.
8.0.0 The ternary operator (? :) is non-associative now; previously it was left-associative.
7.4.0 Relying on the precedence of string concatenation (.) relative to arithmetic addition/subtraction (+ or -) or bitwise shift left/right (<< or >>), i.e. using them together in an unparenthesized expression, is deprecated.
7.4.0 Relying on left-associativity of the ternary operator (? :), i.e. nesting multiple unparenthesized ternary operators, is deprecated.