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Single vs double quotes in PHP: what is the difference?

PHP & the Web 2 min read Full lesson: Hello World in PHP with XAMPP
Short answer

Double-quoted strings interpolate variables and process escape sequences like \n. Single-quoted strings are literal, apart from \\ and \'.

php
$name = "Dom";

echo "Hello $name";     // Hello Dom
echo 'Hello $name';     // Hello $name
echo "Line\nbreak";     // an actual newline
echo 'Line\nbreak';     // the characters backslash-n

#Braces when the variable touches other text

php
echo "That is {$count}x faster";     // without braces, PHP looks for $countx
echo "Title: {$video['title']}";     // required for array access in strings
echo "Name: {$user->name}";

Use braces by default inside double quotes. It costs two characters and removes a whole class of ambiguity.

#Heredoc and nowdoc

For multi-line blocks:

php
$html = <<<HTML
    <div class="card">
      <h2>{$title}</h2>
    </div>
    HTML;

Heredoc (<<<HTML) interpolates like double quotes. Nowdoc (<<<'HTML') is literal like single quotes. Since PHP 7.3 the closing marker can be indented, and that indentation is stripped from every line.

#Does the performance difference matter?

No. The single-quote-is-faster advice is a micro-optimisation from a decade ago that is unmeasurable in any real application. Pick based on whether you want interpolation.

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