PHP Operators

Operators in PHP, and any other prgramming language for that matter, enable us to perfrom tasks on variables and values such as assign, multiply, add, subtract and concatenate them. Operators take the form of symbols (such as + and -) and combinations of symbols (such as ++ and +=).

Operators in PHP work with operands which specify the variables and values that are to be used in the particuilar operation. The number and location of these operands in relation to the operators (i.e. before and/or after the operator) depends on the type of operator in question. Let's take, for example, the following simple expression:

1 + 3;

In this expression we have one operator (the '+') and two operands (the numbers 1 and 3). The '+' operator adds the values of two operands (resulting in a value of 4).

Operators can be combined to create complete expressions:

$myVar = 1 + 3;

In the above example, the assignment operator (=) assigns the result of the addition to the operand represented by the variable $myVar. After evaluating this expression the value of 4 will have been assigned to the variable $myVar.

In this chapter of PHP Essentials we will explore each type of operator and explain how they are used in relation to their operands.

We breifly covered the basic PHP assignment operator in the An Introduction to PHP Variables chapter. We will now look at this and other assignment operators in more detail.

The assignment operator is used to assign a value to a variable and is represented by the equals (=) sign. The assignment operator can also be combined with arithmetic operators to combine an assignment with a mathemtatical operation (for example to multiple one value by another and assigning the result to the variable).

The following table lists the seven assignment operators available in PHP, together with descriptions and examples of their use:

As the name suggests PHP arithmetic operators provide a mechanism for performing mathematical operations:

Arithmetic operators work with two operands, one to the left and the other to the right of the operator. For example:

$var = 1 + 2; // Sets variable $var to the sum of 1 + 2

$var = 3 % 7; // Sets variable $var to the modulus of 3 and 7

$var1 = 10;

$var2 = $var1 / 2; // Sets variable $var2 to the value of 10 divided by 2

The comparison operators provide the ability to compare one value against another and return either a true or false result depending on the status of the match. For example, you might use a comparison operator to check if a variable value matches a particular number, or whether one string is identical to another. PHP provides a wide selection of comparison operators for just about every comparison need.

The comparison operators are used with two operands, one to the left and one to the right of the operator. The following table outlines the PHP comparison operators and provides brief descriptions and examples:

Logical Operators are also known as Boolean Operators because they evaluate parts of an expression and return a true or false value, allowing decisions to be made about how a script should proceed.

The first step to understanding how logical operators work is to construct a sentence rather than to look at a script example right away. Let's assume we need to check some aspect of two variables named $var1 and $var2. Our sentence might read:

If $var is less than 25 AND $var2 is greater than 45 display a message.

Here the logical operator is the "AND" part of the sentence. If we were to express this in PHP we would use the comparison operators we covered earlier together with the && logical operator:

if (($var1 < 25) && ($var2 > 45)
echo 'Our expression is true';

Similarly, if our sentence was to read:

If $var1 is less than 25 OR $var2 is greater than 45 display a message.

Then we would replace the "OR" with the PHP equivalent ||:

if (($var1 < 25) || ($var2 > 45)
echo 'Our expression is true';

Another useful logical operator is the Exclusive Or (XOR) operator. The XOR operator returns true if only one of the expressions evaluates to be true. For example:

If $var is EITHER less than 25 OR greater than 45 display a message

We represent XOR with the keyword xor:

if (($var1 < 25) xor ($var2 > 45)
echo 'Our expression is true';

The final Logical Operator is the NOT operator which simply inverts the result of an expression. The ! character represents the NOT operator and can be used as follows:

(10 > 1)        // returns ''true''

!(10 > 1)       // returns ''false'' because we have inverted the result with the logical NOT

When programming in any language it is not uncommon to need to increment or decrement the value stored in a variable by 1. This could be done long hand:

$myVar = $myVar-1;

A much quicker way, however, is to use the PHP increment and decrement operators. These consist of the operators ++ (to increment) and -- (to decrement) combined with an operand (the name of the variable to which the change is to be applied).

There are two ways of using these operators, pre and post. The pre mode performs the increment or decrement before performing the rest of the expression. For example, you might want to increment the value of a variable before it is assigned to another variable, or used in a calculation. In the post mode the increment or decrement is performed after the expression has been performed. In this instance, you might want the value to be decremented after it has been assigned or used in a calculation.

Whether a pre or post is used depends on whether the operator appears before (for pre), or after (for post) the variable name in the expression. For example --$myVariable or $myVariable++.

The following table outlines the various forms of pre and post increment and decrement operators, together with examples that show how the equivalent task would need to be performed without the increment and decrement operators.

The PHP String concatenation operator is used to combine values to create a string. The concatenation operator is represented by a period/full stop (.) and can be used to build a string from other strings, variables containing non-strings (such as numbers) and even constants:

We will start with the operater in its simplest form concatenating two strings:

echo 'My favorite color is ' . 'blue.';

The above example will display a string that is the result of second string appended to the end of the first string:

My favorite color is blue.

The string concatenation operator can also be used with variables. In the following example the value of the $myString variable is appended to the end of the string:

$myString = "red";

echo 'My favorite color is ' . $myString;

We can also reference constants (see PHP Constants for details on using constants in PHP) when using concatenation:

define (MY_COLOR, "Green");

echo 'My favorite color is ' . MY_COLOR;

The above example will result in the following output:

My favorite color is Green.

We mentioned at the begining of this section that it is also possible to mix numbers and strings in a concatenation operation to create strings. For example we can include the number 6 in our string as follows:

echo 6 . ' is my lucky number';

The above example will create output as follows:

6 is my lucky number

We can also perform a calculation and have the result included in the concatenation:

echo 6 + 5 . ' is my lucky number'

In this example the mathematical operation will be evaluated before the concatenation, thereby producing:

11 is my lucky number

It is important to note an issue when dealing with strings and numbers. While the above works fine because we began the expression with the addition, something very different happens when we have the addition afer the string:

echo 'My Lucky number is ' . 6 + 5;

The above will produce unexpected results (typically it will output just the number 5). The reason for this is because we have asked PHP to take a string (My Lucky number is ), append the number 6 to it (to produce My Lucky number is 6) and then finally arithmetically add the number 5 to the string (which doesn't make a lot of sense). To resolve this issue we need to tell the PHP pre-processor to perform the addition (6 + 5) before performing the concatenation. We can achieve this by surrounding the addition expression in parentheses. Therefore the modified script:

echo 'My Lucky number is ' . (6 + 5);

will now produce the desired output:

My Lucky number is 11

Through the use of parentheses around the addition expression we have changed the operator precedence for that expression.

All of the operators we have looked at so far are similar to those available in other programming and scripting languages. With the execution operator, however, we begin to experience the power of PHP as server side scripting environment. The execution operator allows us to execute a command on the operating system that hosts your web server and PHP module and then capture the output.

You can do anything in an execution operator that you could do as if you were sitting at a terminal window on the computer (within the confines of the user account under which PHP is running). Given this fact, it should not escape your attention that there are potential security risks to this, so this PHP feature should be used with care.

The excecution operator consists of enclosing the command to be executed in back quotes (`). The following example runs the UNIX/Linux uname and id commands to display information about the operating system and user account on which the web server and PHP module are running (note that these command will not work if you are running a Windows based server):

<?php
echo `uname -a` . '<br>';
echo `id`;
?>

This results in the following output in the browser window:

Linux techotopia 2.6.9-42.0.10.ELsmp #1 SMP Tue Feb 27 10:11:19 EST 2007 i686 i686 i386 GNU/Linux

uid=48(apache) gid=48(apache) groups=48(apache)

On a Windows system you could run the dir command to get a listing of files in the current web server directory:

<?php
echo `dir`;
?>