2.4.1. Number of Operands

2.4.2. Operator Precedence

2 + 4 * 3

(2 + 4) * 3

It is possible to write all complex expressions (expressions containing more than a single operator) simply by putting the operands and operators in the appropriate order so that their relative precedence yields the answer you want. Most programmers, however, write the operators in the order that they feel makes the most sense to programmers, and add parentheses to ensure it makes sense to PHP as well. Getting precedence wrong leads to code like:

$x + 2 / $y >= 4 ? $z : $x << $z

This code is hard to read and is almost definitely not doing what the programmer expected it to do.

• Multiplication and division have higher precedence than addition and subtraction.

• Use parentheses for anything else.

2.4.3. Operator Associativity

2 / 2 * 2

The division and multiplication operators have the same precedence, but the result of the expression depends on which operation we do first:

2/(2*2)     // 0.5
(2/2)*2     // 2

The division and multiplication operators are left-associative; this means that in cases of ambiguity, the operators are evaluated from left to right. In this example, the correct result is 2.

2.4.4. Implicit Casting

Table 2-4. Implicit casting rules for binary arithmetic operations

3 . 2.74     // gives the string 32.74 

"9 Lives" - 1;              // 8 (int)
"3.14 Pies" * 2;            // 6.28 (float)
"9 Lives." - 1;             // 8 (float)
"1E3 Points of Light" + 1;  // 1001 (float)

2.4.5. Arithmetic Operators

2.4.6. String Concatenation Operator

$n = 5;
$s = 'There were ' . $n . ' ducks.';
// $s is 'There were 5 ducks'

2.4.7. Autoincrement and Autodecrement Operators

Table 2-5. Autoincrement and autodecrement operations

Table 2-6. Autoincrement with letters

2.4.8. Comparison Operators

Table 2-7. Type of comparision performed by the comparision operators

The comparison operators are:

2.4.9. Bitwise Operators

2.4.10. Logical Operators

Logical AND (&&, and)

The result of the logical AND operation is true if and only if both operands are true; otherwise, it is false. If the value of the first operand is false, the logical AND operator knows that the resulting value must also be false, so the righthand operand is never evaluated. This process is called short-circuiting, and a common PHP idiom uses it to ensure that a piece of code is evaluated only if something is true. For example, you might connect to a database only if some flag is not false:

$result = $flag and mysql_connect( );

The && and and operators differ only in their precedence.

Logical OR (||, or)

The result of the logical OR operation is true if either operand is true; otherwise, the result is false. Like the logical AND operator, the logical OR operator is short-circuited. If the lefthand operator is true, the result of the operator must be true, so the righthand operator is never evaluated. A common PHP idiom uses this to trigger an error condition if something goes wrong. For example:

$result = fopen($filename) or exit( );

The || and or operators differ only in their precedence.

Logical XOR (xor)

The result of the logical XOR operation is true if either operand, but not both, is true; otherwise, it is false.

Logical negation (!)

The logical negation operator returns the Boolean value true if the operand evaluates to false, and false if the operand evaluates to true.

2.4.11. Casting Operators

Table 2-8. PHP casting operators

Casting affects the way other operators interpret a value, rather than changing the value in a variable. For example, the code:

$a = "5";
$b = (int) $a; 

assigns $b the integer value of $a; $a remains the string "5". To cast the value of the variable itself, you must assign the result of a cast back into the variable:

$a = "5"
$a = (int) $a; // now $a holds an integer

Not every cast is useful: casting an array to a numeric type gives 1, and casting an array to a string gives "Array" (seeing this in your output is a sure sign that you've printed a variable that contains an array).

class Person {
  var $name = "Fred";
  var $age  = 35;
}
$o = new Person;
$a = (array) $o;
print_r($a);
Array
(
    [name] => Fred
    [age] => 35
)

You can cast an array to an object to build an object whose properties correspond to the array's keys and values. For example:

$a = array('name' => 'Fred', 'age' => 35, 'wife' => 'Wilma');
$o = (object) $a;
echo $o->name;
Fred

Keys that aren't valid identifiers, and thus are invalid property names, are inaccessible but are restored when the object is cast back to an array.

2.4.12. Assignment Operators

2.4.13. Miscellaneous Operators