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Variables and Operators

Variables in PHP
are identical to variables in most other programming languages. For the uninitiated,
a variable is a name given to an imaginary box into which any value may be
placed. The following statement creates a variable called $testvariable (all
variable names in PHP begin with a dollar sign) and assigns it a value of
3:

$testvariable = 3;

PHP is a loosely typed language. This means that
a single variable may contain any type of data, be it a number, a string of
text, or some other kind of value, and may change types over its lifetime.
So the following statement, if it appears after the statement above, assigns
a new value to our existing $testvariable. In the process,
the variable changes type: where it used to contain a number, it now contains
a string of text:

$testvariable = "Three";

The equals
sign we used in the last two statements is called the assignment
operator, as it is used to assign values to
variables. Other operators may be used to perform various mathematical operations
on values:

$testvariable = 1 + 1;   // Assigns a value of 2
$testvariable = 1 - 1;   // Assigns a value of 0
$testvariable = 2 * 2;   // Assigns a value of 4
$testvariable = 2 / 2;   // Assigns a value of 1

The lines above each end with a comment. Comments are a way to describe what your code is doing—they
insert explanatory text into your code, and tell the PHP interpreter to ignore
it. Comments begin with // and they finish at the
end of the same line. You might be familiar with /* */ style comments in other languages—these work in PHP as
well. I'll be using comments throughout the rest of this book to help explain
what the code I present is doing.

Now, to get back to the four statements above, the operators we used
are called the arithmetic operators, and allow you to add, subtract,
multiply, and divide numbers. Among others, there is also an operator that
sticks strings of text together, called the concatenation operator:

$testvariable = "Hi " . "there!";
// Assigns a value of "Hi there!"

Variables may be used almost anywhere that you use an actual value.
Consider these examples:

$var1 = 'PHP';           // Assigns a value of "PHP" to $var1
$var2 = 5;               // Assigns a value of 5 to $var2
$var3 = $var2 + 1;       // Assigns a value of 6 to $var3
$var2 = $var1;           // Assigns a value of "PHP" to $var2
echo($var1);             // Outputs "PHP"
echo($var2);             // Outputs "PHP"
echo($var3);             // Outputs 6
echo($var1 . ' rules!'); // Outputs "PHP rules!"
echo("$var1 rules!");    // Outputs "PHP rules!"
echo('$var1 rules!');    // Outputs '$var1 rules!'

Notice the last two lines in particular. You can include the name of
a variable right inside a text string, and have the value inserted in its
place if you surround the string with double quotes. This process of
converting variable names to their values is known in technical circles as variable
interpolation. However, as the last line demonstrates,
a string surrounded with single quotes will not interpolate
variable names within the string.