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 series to help
explain what the code I present is doing.
Now, to get back to the four statements above, the operators we
used here 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 especially. 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.
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