PHP: operators language - Comparison Operators

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.

**Comparison Operators**
Example	Name	Result
$a == $b	Equal	`TRUE` if $a is equal to $b.
$a === $b	Identical	`TRUE` if $a is equal to $b, and they are of the same type. (introduced in PHP 4)
$a != $b	Not equal	`TRUE` if $a is not equal to $b.
$a <> $b	Not equal	`TRUE` if $a is not equal to $b.
$a !== $b	Not identical	`TRUE` if $a is not equal to $b, or they are not of the same type. (introduced in PHP 4)
$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.

If you compare an integer with a string, the string is converted to a number. If you compare two numerical strings, they are compared as integers. These rules also apply to the switch statement.


<?php
var_dump(0 == "a"); // 0 == 0 -> true
var_dump("1" == "01"); // 1 == 1 -> true
var_dump("1" == "1e0"); // 1 == 1 -> true

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

For various types, comparison is done according to the following table (in order).

**Comparison with Various Types**
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 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)
array	anything	array is always greater
object	anything	object is always greater

Example #1 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.

Ternary Operator

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

Example #2 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.

Note: Please note that the ternary operator is a statement, and that it doesn't evaluate to a variable, but to the result of a statement. 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 in later PHP versions.

Note: Is 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 #3 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. ?>