Programming PHP, 3rd Edition

Quoting String Constants

There are three ways to write a literal string in your program: using single quotes, double quotes, and the here document (heredoc) format derived from the Unix shell. These methods differ in whether they recognize special escape sequences that let you encode other characters or interpolate variables.

Variable Interpolation

When you define a string literal using double quotes or a heredoc, the string is subject to variable interpolation. Interpolation is the process of replacing variable names in the string with the values of those variables. There are two ways to interpolate variables into strings.

The simpler of the two ways is to put the variable name in a double-quoted string or heredoc:

$who = 'Kilroy';
$where = 'here';
echo "$who was $where";
Kilroy was here

The other way is to surround the variable being interpolated with curly braces. Using this syntax ensures the correct variable is interpolated. The classic use of curly braces is to disambiguate the variable name from surrounding text:

$n = 12;
echo "You are the {$n}th person";
You are the 12th person

Without the curly braces, PHP would try to print the value of the $nth variable.

Unlike in some shell environments, in PHP strings are not repeatedly processed for interpolation. Instead, any interpolations in a double-quoted string are processed first and the result is used as the value of the string:

$bar = 'this is not printed';
$foo = '$bar'; // single quotes
print("$foo");
$bar

Single-Quoted Strings

Single-quoted ...