Example 1: Variable between single quotes
If you use single quotes everything between them will always be treated as part of the string.
$output .= '<div class="tab-pane" id="' . $type . '">";
Example 2: Variable between double quotes (option 1)
If you have a variable that you want to pass in a string you can just put it in there if you use double quotes and de variable is nog 'touching' the other words. It should always have spaces.
$output .= "<p>i would like to $your_text_here with you.</p>";
Example 3: Escaping quotes in a string
Escaping characters in a string can be done by using a \ (backslash) before the character you want to escape.
$output .= "<div class=\"tab-pane\" id=\"example-id\">";
Example 4: Variable between double quotes without spaces next to it
You can place your variable between {} braces if you use double quotes (option 2)
$output .= "<div class=\"tab-pane\" id=\"{$type}\">";
This question was however already answered in Mixing PHP variable with string literal