Dwight Robert. - PHP: Learn PHP in 24 Hours or Less - A Beginners Guide To Learning PHP Programming Now

You also need to learn how to add comments to your PHP codes. As a programmer, you will use comments to add details and descriptions to your work without affecting its output and behavior. The PHP language supports three methods of commenting. These methods are:

• The Shell Method (i.e. #) - In this method, you'll begin your comments with the hashtag symbol. This symbol only works for single-line comments. For example:

# This comment is awesome.

• The C Method (e.g. /* */) - This method is taken from the C language. This option allows you to create multi-line comments. For instance:

/* This comment

spans

multiple lines. */

• The C++ Method (i.e. //) - Use this method when writing a single-line comment. Here's an example:

• // This is a sample.

The variables used in PHP are different from those of compiled languages (e.g. C, C++, Java, etc.). This difference lies in the fact that PHP variables are weakly typed. Simply put, PHP allows you to use variables even without prior declaration. You don't have to declare variables explicitly or assign their type. Thus, you can change the data type of your variables whenever you want.

In the PHP language, you should introduce your variables using the dollar sign (i.e. $). You can use letters, numbers, and underscores when naming your variables. However, you can't use a number as the first character of a variable's name. That means $one_apple is valid while $1_apple isn't.

As mentioned earlier, PHP lets you use undeclared variables. The examples given below will illustrate this idea:

$PI = 3.14159;

$radius = 10;

This code created two variables without declaring their data type. Here, PI belongs to the floating-point type while radius belongs to the integer type.

Important Note: The PHP language is not compatible with global variables. Each variable is local to its scope. If it is created within a function, it will disappear once the function no longer exists.

Indirect Reference

PHP allows you to access a variable using indirect reference. That means you can generate and access a variable by name during runtime. Analyze this example:

$car = BMW;

$$car = Z4;

print $car;

Your screen will display Z4 if you will run this code snippet. The second line of this code accesses the variable named car and changes its value. As you can see, that line has an extra dollar sign. That sign tells the PHP interpreter that you are referring to the value contained by the variable involved. In this example, a new variable named Z4 is generated.

Important Note: There are no limits regarding the number of indirect references that you can use. That means you can place any number of dollar signs before a variable's name.

How to Manage Variables

There are three constructs that you can use to manage your PHP variables. These constructs allow you to verify the existence of certain variables, delete variables, and check their truth values. Let's discuss each language construct in detail:

• isset() - This construct checks whether PHP has declared a particular variable. It will give you true (i.e. a Boolean value) if your chosen variable already exists; otherwise, it will give you false. The script given below will illustrate this concept:

if (isset ($my_name)) {

print '$my_name exists';

}

If the variable named my_name exists, your screen will display $my_name exists. If your code doesn't have that variable, however, you won't get any output.

• unset() - Use this construct to undeclare an existing variable. If there are no references that point to the variable's value, the memory assigned to it will become freed up. Invoking isset() on a variable that you've deleted gives you false. Here's an example:

$gameconsole = PlayStation 4;

unset ($gameconsole);

if (isset ($gameconsole)) {

print '$gameconsole exists';

}

Important Note: You can also use unset() and isset() on object properties and array elements (you'll learn about these later). The syntax that you should use is:

For object properties:

if (isset ($object property)) { }

For array elements:

if isset ($array [offset})) { }

• empty() - With this construct, you can check whether a variable exists or is set to false. While checking the truth value of a variable, empty() will convert the data into Boolean and checks whether it is true or false. Check the following script:

if (empty ($gameconsole)) {

print 'Sorry, this variable doesn't have a value';

}

The Superglobal Variables

In general, you can't use global variables (i.e. variables that you can access from any part of your code) in PHP. However, this scripting language has built-in variables that act like typical global variables. These variables, known as superglobals, are one of the best tools that you can use while writing PHP scripts. Here are some of the superglobals that you will encounter:

• $_ENV[] - This is an array that contains environment variables.
• $_GET[] - This array holds all of the GET variables gathered from the user's web browser.
• $_POST[] - This superglobal is similar to $_GET. The only difference is that $_POST involves POST variables only.
• $_SERVER This kind of array holds the values of web-server variables.

The PHP language supports eight data types. Five of these data types are scalar. The remaining three, however, has unique properties that differentiate them from others. As mentioned earlier, a PHP variable can hold any data type. Keep in mind that PHP variables behave according to the type of data they hold. Here are the data types that you will encounter while using PHP: