Single quoted

echo 'Eat at Joe\'s';

Double quoted

$x = 10;
$y = 20;
echo "x is $x, y is $y"

# PHP 5.4
$person = (object) [
    'name' => 'Joe',
    'age' => 42,
  ];
# PHP 4
echo "name: {$person->name}"

Heredoc

Multi-line replacement:

$x = 'foo';

$text = <<<EOT
First line<br>
Second line<br>
Value: $x
EOT;

echo $text;

Nowdoc (PHP 5.3)

No replacements:

$x = 'foo';

$text = <<<'EOT'
First line<br>
Second line<br>
Value: $x
EOT;

echo $text;