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.
Table 15-4. Comparison Operators
|$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.|
For various types, comparison is done according to the following table (in order).
Table 15-5. 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 15-2. Transcription of standard array comparison
Another conditional operator is the "?:" (or ternary) operator.
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 15-4. Non-obvious Ternary Behaviour
// on first glance, the following appears to output 'true'
// 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.