What is the difference between a single quote and double quote in PHP?

In PHP, single quotes (‘) and double quotes (“) are used to define strings. There are some differences between the two, as follows:

1. Single quotes are slightly faster than double quotes. This is because single quotes do not require the PHP interpreter to parse the string for variables or special characters.
2. Double quotes allow you to embed variables and special characters directly into the string, using the syntax $var or \n respectively. Single quotes do not support this, and you need to use concatenation instead: 'Hello ' . $name . '!'.
3. Single quotes preserve literal strings, while double quotes interpret them. For example, consider the following code:

$name = 'John';
echo 'Hello $name';  // Outputs: Hello $name
echo "Hello $name";  // Outputs: Hello John

In the first example, the string is enclosed in single quotes, so the variable $name is not interpreted and is treated as a literal string. In the second example, the string is enclosed in double quotes, so the variable $name is interpreted and its value is output.

In general, it is recommended to use single quotes unless you need to use the features provided by double quotes (e.g., variable interpolation or special characters). This can help improve the performance of your PHP scripts, as well as make your code more readable.