How do you determine when to use single quotes, double quotes, or no quotes in PHP?

In PHP, single quotes are used to define literal strings where variables are not parsed, double quotes are used to define strings where variables are parsed, and no quotes are used when referring to constants or keywords. To determine which type of quote to use, consider whether variable interpolation is needed in the string. If variables need to be evaluated within the string, use double quotes. If no variable interpolation is needed, use single quotes. Example PHP code snippet: ``` $name = 'John'; echo 'Hello, ' . $name; // Output: Hello, John $age = 25; echo "I am $age years old"; // Output: I am 25 years old define('GREETING', 'Welcome'); echo GREETING; // Output: Welcome ```