Sams Teach Yourself PHP, MySQL and Apache All in One (2012)

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Now that you have the principles out of the way, it’s time to take a tour of the operators commonly used in PHP programming.

The Assignment Operator

You have seen the assignment operator in use each time a variable was declared in an example; the assignment operator consists of the single character: =. The assignment operator takes the value of the right-side operand and assigns it to the left-side operand:

$name = "Jimbo";

The variable $name now contains the string "Jimbo". This construct is also an expression. Although it might seem at first glance that the assignment operator simply changes the variable $name without producing a value, a statement that uses the assignment operator always resolves to a copy of the value of the right operand. Therefore

echo $name = "Jimbo";

prints the string "Jimbo" to the browser while it also assigns the value "Jimbo" to the $name variable.

Arithmetic Operators

The arithmetic operators do exactly what you would expect—they perform arithmetic operations. Table 5.2 lists these operators along with examples of their usage and results.

Table 5.2 Arithmetic Operators

The addition operator adds the right-side operand to the left-side operand. The subtraction operator subtracts the right-side operand from the left-side operand. The division operator divides the left-side operand by the right-side operand. The multiplication operator multiplies the left-side operand by the right-side operand. The modulus operator returns the remainder of the left-side operand divided by the right-side operand.

The Concatenation Operator

The concatenation operator is represented by a single period (.). Treating both operands as strings, this operator appends the right-side operand to the left-side operand. So

"hello"." world"

returns

"hello world"

Note that the resulting space between the words occurs because there is a leading space in the second operand (" world" rather than "world"). The concatenation operator literally smashes together two strings without adding any padding. So, if you try to concatenate two strings without leading or trailing spaces, such as

"hello"."world"

you will get this as your result:

"helloworld"

Regardless of the data types of the operands used with the concatenation operator, they are treated as strings, and the result is always of the string type. You will encounter concatenation frequently throughout this book when the results of an expression of some kind must be combined with a string, as in the following:

$cm = 212;
echo "the width is ".($cm/100)." meters";

Combined Assignment Operators

Although there is only one true assignment operator, PHP provides a number of combination operators that transform the left-side operand and return a result while also modifying the original value of the variable. As a rule, operators use operands but do not change their original values, but combined assignment operators break this rule. A combined assignment operator consists of a standard operator symbol followed by an equal sign. Combination assignment operators save you the trouble of using two operators in two different steps within your script. For example, if you have a variable with a value of 4, and you want to increase this value to 4 more, you might see this:

$x = 4;
$x = $x + 4; // $x now equals 8

However, you can also use a combination assignment operator (+=) to add and return the new value, as shown here:

$x = 4;
$x += 4; // $x now equals 8

Each arithmetic operator, as well as the concatenation operator, also has a corresponding combination assignment operator. Table 5.3 lists these new operators and shows an example of their usage.

Table 5.3 Some Combined Assignment Operators

Each of the examples in Table 5.3 transforms the value of $x using the value of the right-side operand. Subsequent uses of $x will refer to the new value. For example

$x = 4;
$x += 4; // $x now equals 8
$x += 4; // $x now equals 12
$x -= 3; // $x now equals 9

These operators are used throughout the scripts in the book. You will frequently see the combined concatenation assignment operator when you begin to create dynamic text. Looping through a script and adding content to a string, such as dynamically building the HTML markup to represent a table, is a prime example of the use of a combined assignment operator.

Automatically Incrementing and Decrementing an Integer Variable

When coding in PHP, you will often find it necessary to increment or decrement a variable that is an integer type. You usually need to do this when you are counting the iterations of a loop. You have already learned two ways of doing this—either by incrementing the value of $x using the addition operator

$x = $x + 1; // $x is incremented by 1

or by using a combined assignment operator

$x += 1; // $x is incremented by 1

In both cases, the new value is assigned to $x. Because expressions of this kind are common, PHP provides some special operators that allow you to add or subtract the integer constant 1 from an integer variable, assigning the result to the variable itself. These are known as the post-incrementand post-decrement operators. The post-increment operator consists of two plus symbols appended to a variable name:

$x++; // $x is incremented by 1

This expression increments the value represented by the variable $x by 1. Using two minus symbols in the same way decrements the variable:

$x--; // $x is decremented by 1

If you use the post-increment or post-decrement operators in conjunction with a conditional operator, the operand is modified only after the first operation has finished:

$x = 3;
$y = $x++ + 3;

In this instance, $y first becomes 6 (the result of 3 + 3), and then $x is incremented.

In some circumstances, you might want to increment or decrement a variable in a test expression before the test is carried out. PHP provides the pre-increment and pre-decrement operators for this purpose. These operators behave in the same way as the post-increment and post-decrement operators, but they are written with the plus or minus symbols preceding the variable:

++$x; // $x is incremented by 1
--$x; // $x is decremented by 1

If these operators are used as part of a test expression, incrementing occurs before the test is carried out. For example, in the next fragment, $x is incremented before it is tested against 4:

$x = 3;
++$x < 4; // false

The test expression returns false because 4 is not smaller than 4.

Comparison Operators

Comparison operators perform comparative tests using their operands and return the Boolean value true if the test is successful or false if the test fails. This type of expression is useful when using control structures in your scripts, such as if and while statements. This book covers if andwhile statements in Chapter 6, “Flow Control Functions in PHP.”

For example, to test whether the value contained in $x is smaller than 5, you can use the less-than operator as part of your expression:

$x < 5

If $x contains the value 3, this expression has the value true. If $x contains 7, the expression resolves to false.

Table 5.4 lists the comparison operators.

Table 5.4 Comparison Operators

These operators are most commonly used with integers or doubles, although the equivalence operator is also used to compare strings. Be very sure to understand the difference between the == and = operators. The == operator tests equivalence, whereas the = operator assigns value. Also, remember that === tests equivalence with regard to both value and type.

Creating Complex Test Expressions with the Logical Operators

Logical operators test combinations of Boolean values. For example, the or operator, which is indicated by two pipe characters (||) or simply the word or, returns the Boolean value true if either the left or the right operand is true:

true || false

This expression returns true.

The and operator, which is indicated by two ampersand characters (&&) or simply the word and, returns the Boolean value true only if both the left and right operands are true:

true && false

This expression returns the Boolean value false. It’s unlikely that you will use a logical operator to test Boolean constants because it makes more sense to test two or more expressions that resolve to a Boolean. For example

($x > 2) && ($x < 15)

returns the Boolean value true if $x contains a value that is greater than 2 and smaller than 15. Parentheses are used when comparing expressions to make the code easier to read and to indicate the precedence of expression evaluation. Table 5.5 lists the logical operators.

Table 5.5 Logical Operators

You might wonder why are there two versions of both the or and the and operators, and that’s a good question. The answer lies in operator precedence, which we examine next.

Operator Precedence

When you use an operator within an expression, the PHP engine usually reads your expression from left to right. For complex expressions that use more than one operator, though, the PHP engine could be led astray without some guidance. First, consider a simple case:

4 + 5

There’s no room for confusion here; PHP simply adds 4 to 5. But what about the following fragment, with two operators:

4 + 5 * 2

This presents a problem. Should PHP find the sum of 4 and 5, and then multiply it by 2, providing the result 18? Or does it mean 4 plus the result of 5 multiplied by 2, resolving to 14? If you were simply to read from left to right, the former would be true. However, PHP attaches different precedence to different operators, and because the multiplication operator has higher precedence than the addition operator, the second solution to the problem is the correct one: 4 plus the result of 5 multiplied by 2.

However, you can override operator precedence by putting parentheses around your expressions. In the following fragment, the addition expression is evaluated before the multiplication expression:

(4 + 5) * 2

Whatever the precedence of the operators in a complex expression, it is a good idea to use parentheses to make your code clearer and to save you from bugs such as applying sales tax to the wrong subtotal in a shopping cart situation. The following is a list of the operators covered in this chapter in precedence order (those with the highest precedence listed first):

++, --, (cast)
/, *, %
+, -
<, <=, =>, >
==, ===, !=
&&
||
=, +=, -=, /=, *=, %=, .=
and
xor
or

As you can see, or has a lower precedence than ||, and and has a lower precedence than &&, so you can use the lower-precedence logical operators to change the way a complex test expression is read. In the following fragment, the two expressions are equivalent, but the second is much easier to read:

$x and $y || $z

$x && ($y || $z)

Taking it one step further, the following fragment is easier still:

$x and ($y or $z)

However, all three examples are equivalent.

The order of precedence is the only reason that both && and and are available in PHP. The same is true of || and or. In most circumstances, the use of parentheses makes for clearer code and fewer bugs than code that takes advantage of the difference in precedence of these operators. This book tends to use the more common || and && operators, and relies on parenthetical statements to set specific operator precedence.

Constants

Variables offer a flexible way of storing data because you can change their values and the type of data they store at any time during the execution of your scripts. However, if you want to work with a value that must remain unchanged throughout your script’s execution, you can define and use a constant. You must use PHP’s built-in define() function to create a constant, which subsequently cannot be changed unless you specifically define() it again. To use the define() function, place the name of the constant and the value you want to give it within parentheses and separated by a comma:

define("YOUR_CONSTANT_NAME", 42);

The value you want to set can be a number, a string, or a Boolean. By convention, the name of the constant should be in capital letters. Constants are accessed with the constant name only; no dollar symbol is required. Listing 5.4 shows you how to define and access a constant.

Listing 5.4 Defining and Accessing a Constant