In what situations should developers use single quotes versus double quotes in PHP, especially when dealing with variables or special characters?

Developers should use single quotes in PHP when dealing with simple strings that do not contain variables or special characters, as single quotes are faster and more memory-efficient. Double quotes should be used when including variables or special characters that need to be interpreted by PHP. This is because variables inside single quotes will not be parsed by PHP, whereas they will be parsed inside double quotes.

// Example of using single quotes for simple strings
$myString = 'Hello, world!';

// Example of using double quotes to include variables
$name = 'Alice';
$greeting = "Hello, $name!";
echo $greeting;