Comparison Operators

Comparison operators, as their name implies, allow you to compare two values. You may also be interested in viewing the type comparison tables, as they show examples of various type related comparisons.
Example
Name
Result
$a == $b
Equal
ENTITY if 
$a is equal to 
$b after type juggling.
$a === $b
Identical
ENTITY if 
$a is equal to 
$b, and they are of the same type.
$a != $b
Not equal
ENTITY if 
$a is not equal to 
$b after type juggling.
$a ENTITYENTITY $b
Not equal
ENTITY if 
$a is not equal to 
$b after type juggling.
$a !== $b
Not identical
ENTITY if 
$a is not equal to 
$b, or they are not of the same type.
$a ENTITY $b
Less than
ENTITY if 
$a is strictly less than 
$b.
$a ENTITY $b
Greater than
ENTITY if 
$a is strictly greater than 
$b.
$a ENTITY= $b
Less than or equal to
ENTITY if 
$a is less than or equal to 
$b.
$a ENTITY= $b
Greater than or equal to
ENTITY if 
$a is greater than or equal to 
$b.
$a ENTITY=ENTITY $b
Spaceship
An 
int less than, equal to, or greater than zero when 
$a is less than, equal to, or greater than 
$b, respectively.
Comparison Operators
If both operands are numeric strings, or one operand is a number and the other one is a numeric string, then the comparison is done numerically. These rules also apply to the switch statement. The type conversion does not take place when the comparison is 
=== or 
!== as this involves comparing the type as well as the value.
[!WARNING]
Prior to PHP 8.0.0, if a 
string is compared to a number or a numeric string then the 
string was converted to a number before performing the comparison. This can lead to surprising results as can be seen with the following example:
<?php
var_dump(0 == "a");
var_dump("1" == "01");
var_dump("10" == "1e1");
var_dump(100 == "1e2");
switch ("a") {
case 0:
    echo "0";
    break;
case "a":
    echo "a";
    break;
}
?>
    
ENTITY
bool(true)
bool(true)
bool(true)
bool(true)
0
ENTITY
bool(false)
bool(true)
bool(true)
bool(true)
a
Comparison Operators
<?php
// Integers
echo 1 <=> 1, ' '; // 0
echo 1 <=> 2, ' '; // -1
echo 2 <=> 1, ' '; // 1
// Floats
echo 1.5 <=> 1.5, ' '; // 0
echo 1.5 <=> 2.5, ' '; // -1
echo 2.5 <=> 1.5, ' '; // 1
// Strings
echo "a" <=> "a", ' '; // 0
echo "a" <=> "b", ' '; // -1
echo "b" <=> "a", ' '; // 1
echo "a" <=> "aa", ' ';  // -1
echo "zz" <=> "aa", ' '; // 1
// Arrays
echo [] <=> [], ' ';               // 0
echo [1, 2, 3] <=> [1, 2, 3], ' '; // 0
echo [1, 2, 3] <=> [], ' ';        // 1
echo [1, 2, 3] <=> [1, 2, 1], ' '; // 1
echo [1, 2, 3] <=> [1, 2, 4], ' '; // -1
// Objects
$a = (object) ["a" => "b"];
$b = (object) ["a" => "b"];
echo $a <=> $b, ' '; // 0
$a = (object) ["a" => "b"];
$b = (object) ["a" => "c"];
echo $a <=> $b, ' '; // -1
$a = (object) ["a" => "c"];
$b = (object) ["a" => "b"];
echo $a <=> $b, ' '; // 1
// not only values are compared; keys must match
$a = (object) ["a" => "b"];
$b = (object) ["b" => "b"];
echo $a <=> $b, ' '; // 1
?>
   
For various types, comparison is done according to the following table (in order).
Type of Operand 1
Type of Operand 2
Result
null or 
string
string
Convert ENTITY to "", numerical or lexical comparison
bool or 
null
anything
Convert both sides to 
bool, ENTITY ENTITY ENTITY
object
object
Built-in classes can define its own comparison, different classes are incomparable, same class see Object Comparison
string, 
resource, 
int or 
float
string, 
resource, 
int or 
float
Translate strings and resources to numbers, usual math
array
array
Array with fewer members is smaller, if key from operand 1 is not found in operand 2 then arrays are incomparable, otherwise - compare value by value (see following example)
object
anything
object is always greater
array
anything
array is always greater
Comparison with Various Types {#language.operators.comparison.types}
Boolean/null comparison
<?php
// Bool and null are compared as bool always
var_dump(1 == TRUE);  // TRUE - same as (bool) 1 == TRUE
var_dump(0 == FALSE); // TRUE - same as (bool) 0 == FALSE
var_dump(100 < TRUE); // FALSE - same as (bool) 100 < TRUE
var_dump(-10 < FALSE);// FALSE - same as (bool) -10 < FALSE
var_dump(min(-100, -10, NULL, 10, 100)); // NULL - (bool) NULL < (bool) -100 is FALSE < TRUE
?>
   
Transcription of standard array comparison
<?php
// Arrays are compared like this with standard comparison operators as well as the spaceship operator.
function standard_array_compare($op1, $op2)
{
    if (count($op1) < count($op2)) {
        return -1; // $op1 < $op2
    } elseif (count($op1) > count($op2)) {
        return 1; // $op1 > $op2
    }
    foreach ($op1 as $key => $val) {
        if (!array_key_exists($key, $op2)) {
            return 1;
        } elseif ($val < $op2[$key]) {
            return -1;
        } elseif ($val > $op2[$key]) {
            return 1;
        }
    }
    return 0; // $op1 == $op2
}
?>
   
[!WARNING]
Because of the way 
floats are represented internally, you should not test two 
floats for equality.
See the documentation for 
float for more information.
[!NOTE]
Be aware that PHP's type juggling is not always obvious when comparing values of different types, particularly comparing ENTITYs to ENTITYs or ENTITYs to ENTITYs. It is therefore generally advisable to use 
=== and 
!== comparisons rather than 
== and 
!= in most cases.
[!NOTE]
Encountering recursion during comparison, for example when comparing an array or object that contains a reference to itself, throws an Error as of PHP 8.4.0; previously, it resulted in a fatal error.
Incomparable Values
While identity comparison (
=== and 
!==) can be applied to arbitrary values, the other comparison operators should only be applied to comparable values. The result of comparing incomparable values is undefined, and should not be relied upon.
[!NOTE]
As of PHP 8.5.0, loosely comparing an 
object that is not comparable (such as an enum, 
CurlHandle, or another internal class) against a 
bool using 
== or 
!= consistently evaluates as if the object were cast to 
bool, that is 
(bool) $object. Because such an object always casts to ENTITY, it is equal to ENTITY and not equal to ENTITY. Prior to PHP 8.5.0, this only applied when comparing against a boolean literal (
$object == true); comparing against a boolean value held in a variable always returned ENTITY. See the migration guide for details.
ENTITY
strcasecmp, 
strcmp, Array operators, Types
Ternary Operator
Another conditional operator is the "?:" (or ternary) operator.
Assigning a default value
<?php
// Example usage for: Ternary Operator
$action = (empty($_POST['action'])) ? 'default' : $_POST['action'];
// The above is identical to this if/else statement
if (empty($_POST['action'])) {
    $action = 'default';
} else {
    $action = $_POST['action'];
}
?>
    
The expression 
(expr1) ? (expr2) : (expr3) evaluates to <expr2> if <expr1> evaluates to ENTITY, and <expr3> if <expr1> evaluates to ENTITY.
It is possible to leave out the middle part of the ternary operator. Expression 
expr1 ?: expr3 evaluates to the result of <expr1> if <expr1> evaluates to ENTITY, and <expr3> otherwise. <expr1> is only evaluated once in this case.
[!NOTE]
Please note that the ternary operator is an expression, and that it doesn't evaluate to a variable, but to the result of an expression. This is important to know if you want to return a variable by reference. The statement 
return $var == 42 ? $a : $b; in a return-by-reference function will therefore not work and a warning is issued.
[!NOTE]
It is recommended to avoid "stacking" ternary expressions. PHP's behaviour when using more than one unparenthesized ternary operator within a single expression is non-obvious compared to other languages. Indeed prior to PHP 8.0.0, ternary expressions were evaluated left-associative, instead of right-associative like most other programming languages. Relying on left-associativity is deprecated as of PHP 7.4.0. As of PHP 8.0.0, the ternary operator is non-associative.
Non-obvious Ternary Behaviour
<?php
// on first glance, the following appears to output 'true'
echo (true ? 'true' : false ? 't' : 'f');
// however, the actual output of the above is 't' prior to PHP 8.0.0
// this is because ternary expressions are left-associative
// the following is a more obvious version of the same code as above
echo ((true ? 'true' : false) ? 't' : 'f');
// here, one can see that the first expression is evaluated to 'true', which
// in turn evaluates to (bool) true, thus returning the true branch of the
// second ternary expression.
?>
     
[!NOTE]
Chaining of short-ternaries (
?:), however, is stable and behaves reasonably. It will evaluate to the first argument that evaluates to a non-falsy value. Note that undefined values will still raise a warning.
Short-ternary chaining
<?php
echo 0 ?: 1 ?: 2 ?: 3, PHP_EOL; //1
echo 0 ?: 0 ?: 2 ?: 3, PHP_EOL; //2
echo 0 ?: 0 ?: 0 ?: 3, PHP_EOL; //3
?>
     
Null Coalescing Operator
Another useful shorthand operator is the "??" (or null coalescing) operator.
Assigning a default value
<?php
// Example usage for: Null Coalesce Operator
$action = $_POST['action'] ?? 'default';
// The above is identical to this if/else statement
if (isset($_POST['action'])) {
    $action = $_POST['action'];
} else {
    $action = 'default';
}
?>
    
The expression 
(expr1) ?? (expr2) evaluates to <expr2> if <expr1> is ENTITY, and <expr1> otherwise.
In particular, this operator does not emit a notice or warning if the left-hand side value does not exist, just like 
isset. This is especially useful on array keys.
[!NOTE]
Please note that the null coalescing operator is an expression, and that it doesn't evaluate to a variable, but to the result of an expression. This is important to know if you want to return a variable by reference. The statement 
return $foo ?? $bar; in a return-by-reference function will therefore not work and a warning is issued.
[!NOTE]
The null coalescing operator has low precedence. That means if mixing it with other operators (such as string concatenation or arithmetic operators) parentheses will likely be required.
<?php
// Raises a warning that $name is undefined.
print 'Mr. ' . $name ?? 'Anonymous';
// Prints "Mr. Anonymous"
print 'Mr. ' . ($name ?? 'Anonymous');
?>
   
[!NOTE]
Please note that the null coalescing operator allows for simple nesting:
Nesting null coalescing operator
<?php
$foo = null;
$bar = null;
$baz = 1;
$qux = 2;
echo $foo ?? $bar ?? $baz ?? $qux; // outputs 1
?>