What is the difference between using double and single quotes in programming languages?

In PHP, single quotes ('') and double quotes ("") are used to create strings, but there are important differences:

• In double quotes, special characters (e.g., \n, \t) are interpreted, and variables are embedded directly into the string.
• In single quotes, the string is taken literally: special characters are not processed (except ' and \), and variables are not embedded.

Example:

$name = "Ivan";

echo "Hello, $name\n"; // Outputs: Hello, Ivan (and a newline)
echo 'Hello, $name\n'; // Outputs: Hello, $name\n

Use single quotes if you do not need to interpret variables and special characters — this is slightly more efficient. If variable interpolation or special characters are needed, use double quotes.