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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.

If you compare a number with a string or the comparison involves numerical strings, then each string is converted to a number and the comparison performed 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.

<?php
var_dump(0 == "a" // 0 == 0 -> true
var_dump("1" == "01" // 1 == 1 -> true
var_dump("10" == "1e1" // 10 == 10 -> true
var_dump(100 == "1e2" // 100 == 100 -> true

switch ("a") {
case 0:
    echo "0";
    break;
case "a": // never reached because "a" is already matched with 0
    echo "a";
    break;
}
?>

<?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
 
// only values are compared
$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).

Example #1 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
?>

Example #2 Transcription of standard array comparison

<?php
// Arrays are compared like this with standard comparison operators
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, $op2)) {
            return null; // uncomparable
        } elseif ($val < $op2[$key]) {
            return -1;
        } elseif ($val > $op2[$key]) {
            return 1;
        }
    }
    return 0; // $op1 == $op2
}
?>

See also strcasecmp(), strcmp(), Array operators, and the manual section on Types.

Warning

Comparison of floating point numbers

Because of the way floats are represented internally, you should not test two floats for equality.

See the documentation for float for more information.

Ternary Operator

Another conditional operator is the "?:" (or ternary) operator.

Example #3 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 TRUE, and expr3 if expr1 evaluates to FALSE.

Since PHP 5.3, it is possible to leave out the middle part of the ternary operator. Expression expr1 ?: expr3 returns expr1 if expr1 evaluates to TRUE, and expr3 otherwise.

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 that you avoid "stacking" ternary expressions. PHP's behaviour when using more than one ternary operator within a single statement is non-obvious: Example #4 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' // this is because ternary expressions are evaluated from left to right // the following is a more obvious version of the same code as above echo ((true ? 'true' : false) ? 't' : 'f' // here, you 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. ?>

Null Coalescing Operator

Further exists the "??" (or null coalescing) operator, available as of PHP 7.

Example #5 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 NULL, and expr1 otherwise.

In particular, this operator does not emit a notice 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: Please note that the null coalescing operator allows for simple nesting: Example #6 Nesting null coalescing operator <?php $foo = null; $bar = null; $baz = 1; $qux = 2; echo $foo ?? $bar ?? $baz ?? $qux; // outputs 1 ?>

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