Variable and constants
In programming, a variable means a value holder. A variable can hold the same value or the value it holds can get changed during the runtime of a program.
03.$greeting = ‘Welcome’;
05.$name = ‘Ram’;
06.echo “$greeting $name”;
07.echo ‘<br />’;
09.$name = ‘Ramu’;
10.echo “$greeting $name”;
11.echo ‘<br />’;
13.$name = ‘Raj’;
14.echo “$greeting $name”;
15.echo ‘<br />’;
Run above script in web browser and you would see following three lines.
In this script both $greeting and $name are variables. You can see that $greeting holds the same value throughout the script while value of $name gets changed. We print <br /> tags just to have line breaks in the browser.
In PHP, all variables should start with $ sign. The part after $ sign is called variable name.
- Variable names can only contain letters, numbers and underscores.
- A variable name should only start with a letter or an underscore.
- Variable names are case-sensitive. That means $Greeting and $greeting are two different variables.
Some good practices in variable naming.
1.$firstName // Correct
2.$first_name // Correct
3.$firstName1 // Correct
4.$_firstName // Correct
5.$first-name // Wrong (Hyphen is not allowed)
6.$4firstName // Wrong (Starts with a number)
All the variables considered so far could contain only one value at a time. Arrays provide a way to store a set of values under one variable.
PHP provides a set of predefined variables. Most of them are arrays. Their availability and values are based on the context. For an example $_POST contains all the values submitted via a web form that used post method. Refer PHP manual for more information on predefined variables.
As name describes, constants hold values that don’t get changed during the runtime of a script. Same naming rules apply for constants except that $ sign is not used when defining constants. To give an emphasis to constants, it’s a common practice to define constant names in upper-case.
2.echo SITE_URL; // Would print http://www.example.com
Once defined, a new value cannot be assigned to a constant.
2.echo SITE_URL; // Would still print http://www.example.com
It includes the following topics –
- Data Types
- Special Characters