Single vs double quotes in PHP

PHP Strings

Will display things almost completely as is.
Variables and most escape sequences will not be interpreted.
The exception is that to display a literal single quote, you can escape it with a back slash \',and to display a back slash, you can escape it with another backslash \\.

Example:

$variable = 10;
echo 'Value is $variable';

Output:

Value is $variable

Will display a host of escaped characters (including some regexes), and variables in the strings will be evaluated.
An important point here is that you can use curly braces to isolate the name of the variable you want evaluated.

Example:

$variable = 10;
echo "Value is $variable";

or

$variable = 10;
echo "Value is {$variable}";

Output:

Value is 10

When complex variable interpolation (array/object member accessing) is required it's better to use curly braces.

Example:

echo "Value is {$arr[0]}";
echo "ID is {$obj->id}";

Variable interpolation with curly braces can also contain complex expressions within accessors.

Example:

echo "Value is {$arr[max($min, $max)]}";

Works like double quoted strings.
It starts with <<<.
After this operator, an identifier is provided, then a newline. The string itself follows, and then the same identifier again to close the quotation.
You don't need to escape quotes in this syntax.

Works essentially like single quoted strings.
The difference is that not even single quotes or backslashes have to be escaped.
A nowdoc is identified with the same <<< sequence used for heredocs, but the identifier which follows is enclosed in single quotes, e.g. <<<'EOT'. No parsing is done in nowdoc.