PHP and MySQL Web Development 4th Ed-tqw-_darksiderg

Accessing Form Variables
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This is equivalent to the first statement shown in this section. Either format is valid, and
which one you use is a matter of personal taste.This process, replacing a variable with its
contents within a string, is known as interpolation.
Note that interpolation is a feature of double-quoted strings only.You cannot place
variable names inside a single-quoted string in this way. Running the following line of
code
echo ‘$tireqty tires’;
simply sends “$tireqty tires” to the browser.Within double quotation marks,
the variable name is replaced with its value.Within single quotation marks, the variable
name or any other text is sent unaltered.
Variables and Literals
The variables and strings concatenated together in each of the echo statements in the
sample script are different types of things.Variables are symbols for data.The strings are
data themselves.When we use a piece of raw data in a program like this, we call it a literal
to distinguish it from a variable. $tireqty is a variable, a symbol that represents the
data the customer typed in. On the other hand, ‘ tires’ is a literal.You can take
it at face value.Well, almost. Remember the second example in the preceding section?
PHP replaced the variable name $tireqty in the string with the value stored in the
variable.
Remember the two kinds of strings mentioned already: ones with double quotation
marks and ones with single quotation marks. PHP tries to evaluate strings in double
quotation marks, resulting in the behavior shown earlier. Single-quoted strings are treated
as true literals.
There is also a third way of specifying strings using the heredoc syntax (