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PHP Variables & Constants - Basics
A variable is a place in the computer’s memory where you can store some data. The stored value can change, and is often as the result of some calculation or processing having taken place.
A variable in PHP is identified by the first character being a dollar sign, ‘$’. Variables do not have to be declared before using them, and they have no value until assigned one. The process of assigning a value to the variable also usually defines what type of variable it is, e.g. text or numeric.
This process of implicitly defining a variable means that if the value is changed, it can be implicitly re-defined. The following line of code shows a variable called $num, and assigns it a value of 17. This implicitly defines it as an integer (whole number).
<? $num=17; ?>
To re-define this variable, so it contains text (a string of characters), we use the following code:
<? $num="University of Wolverhampton"; ?>
Note, that this would not be a meaningful name for this variable. Perhaps a more meaningful name might be $university. The process of re-defining variables can both be flexible and dangerous, if not used with some care.
Look at the following example which uses both a string and an integer variable.
<? $students = 212; $module = "CP1082"; echo "The class $module has $students enrolled on it"; ?>
This produces:
The class CP1082 has 212 enrolled on it
NOTE: - a common error is to use an undefined variable in an expression.
Types of Variable
There are four type of scalar (can contain a single value at any given time) variable supported by PHP. These are:
- Boolean
- Float
- Integer
- String
PHP also supports compound types (array or object). These can contain a number of scalar values, such as the days of the week, the scores of students in an exam etc.
Variable names follow the same rules as other labels in PHP. A valid variable name starts with a letter or underscore, followed by any number of letters, numbers, or underscores. The underscore is used instead of a space (space in variables names is not permitted in PHP).
For example, if you chose the variable name '$my age' it would not be permitted. The variable '$my_age' would be.
Boolean
These are very simple variables, in that they can only hold the values true or false. They are often used to determine what action should be taken - according to their value.
Integer
A whole number, sometimes known as a counting number. Note, these can be positive or negative, e.g. -37, 5, 4732, -162, 0, 56
<? // This is an integer $distance = 42; ?>
Float
A number which has a fractional part. Note this may be a decimal, or represented in exponential notation (scientific format).
<? // This is a float $distance = 42.0; // This is a float equal to 1120 $num = 1.12e3; //This is a float equal to 0.02 $num = 2e-2; ?>
String
We have already seen numerous examples of string variables on previous pages. A string can contain a single character, or numerous characters.
<? $initial="B"; $surname="Riordan"; echo $initial." ".$surname; ?>
Constants
If variables are values which may 'vary' during the life of a script, then a constant is a value which remains 'constant' (does not vary) during the life of that script. Therefore, it is assigned a value which is used throughout the script. Examples would be the number of hours in a day, the number of students on a course, the rate of VAT etc.
<?
define ("PI", 3.142);
// output the value of twice PI
echo (2 * PI);
?>
This produces:
6.284
NOTE: - Conventions are for constants to be UPPERCASE
Note the syntax of the definition of PI, and the fact that unlike variables, constants do not have a '$' as their first character.
Constants are much underused by new programmers. If used to define the number of lines used on a web page, it can be altered once, without having to make numerous changes throughout the code.
