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 | TRUE if $a is equal to $b after type juggling. |
| $a === $b | Identical |
TRUE if $a is equal to $b, and they are of the same
type.
|
| $a != $b | Not equal | TRUE if $a is not equal to $b after type juggling. |
| $a <> $b | Not equal | TRUE if $a is not equal to $b after type juggling. |
| $a !== $b | Not identical |
TRUE if $a is not equal to $b, or they are not of the same
type.
|
| $a < $b | Less than | TRUE if $a is strictly less than $b. |
| $a > $b | Greater than | TRUE if $a is strictly greater than $b. |
| $a <= $b | Less than or equal to | TRUE if $a is less than or equal to $b. |
| $a >= $b | Greater than or equal to | TRUE if $a is greater than or equal to $b. |
| $a <=> $b | Spaceship | An integer less than, equal to, or greater than zero when $a is respectively less than, equal to, or greater than $b. Available as of PHP 7. |
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 <=> $b; // 1
?>
For various types, comparison is done according to the following table (in order).
| Type of Operand 1 | Type of Operand 2 | Result |
|---|---|---|
| null or string | string | Convert NULL to "", numerical or lexical comparison |
| bool or null | anything | Convert both sides to bool, FALSE < TRUE |
| object | object | Built-in classes can define its own comparison, different classes are uncomparable, same class - compare properties the same way as arrays (PHP 4), PHP 5 has its own explanation |
| string, resource or number | string, resource or number | Translate strings and resources to numbers, usual math |
| array | array | Array with fewer members is smaller, if key from operand 1 is not found in operand 2 then arrays are uncomparable, otherwise - compare value by value (see following example) |
| object | anything | object is always greater |
| array | anything | array is always greater |
Example #1 Boolean/null comparison
<?php
// Bool and null are compared as bool always
var_dump(1 == TRUE); // TRUE - same as (bool)1 == TRUE
var_dump(0 == FALSE); // TRUE - same as (bool)0 == FALSE
var_dump(100 < TRUE); // FALSE - same as (bool)100 < TRUE
var_dump(-10 < FALSE);// FALSE - same as (bool)-10 < FALSE
var_dump(min(-100, -10, NULL, 10, 100)); // NULL - (bool)NULL < (bool)-100 is FALSE < TRUE
?>
Example #2 Transcription of standard array comparison
<?php
// Arrays are compared like this with standard comparison operators
function standard_array_compare($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.
Because of the way floats are represented internally, you should not test two floats for equality.
See the documentation for float for more information.
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'];
}
?>
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.
?>
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';
}
?>
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
?>