Difference Between Single And Double Quotes In PHP

In PHP, single quotes (') and double quotes (") are used to denote strings, but they work slightly differently.

Single Quotes:

Strings within single quotes are treated almost literally. The only special characters in it are the backslash (\) and the single-quote itself (').
Variable names inside a single-quoted string are not parsed. They will be treated as regular text, not the value of the variable.

Example:

$example = 'world';
echo 'Hello, $example!';  // Output: Hello, $example!

Double Quotes:

Strings within double quotes interpret more escape sequences and they do variable parsing.
Special characters like newline (\n), tab (\t), etc. are recognized and processed within double quotes.
Variables inside a double-quoted string will be parsed, which means PHP will replace the variable with its value.

Example:

$example = 'world';
echo "Hello, $example!";  // Output: Hello, world!

Here's an example showing the difference with escape sequences:

echo 'Hello\tWorld';  // Output: Hello\tWorld
echo "Hello\tWorld";  // Output: Hello    World

The first line will literally print out "Hello\tWorld", while the second line will print out "Hello", followed by a tab, and then "World".

In terms of performance, single quotes can be a tiny bit faster because the string is not parsed for special characters or variables, but the difference is so small that it's not really a factor you should worry about in choosing which one to use. The choice between single and double quotes should be based on the needs of the string you're working with.

PHP single vs. double quotes comparison:
- In PHP, both single (' ') and double (" ") quotes can be used to define strings. Single quotes are more rigid, while double quotes offer more features like variable interpolation.
```
$singleQuoted = 'This is a single-quoted string.';
$doubleQuoted = "This is a double-quoted string.";
```
String interpolation and variable substitution in single and double quotes in PHP:
- Double quotes allow for string interpolation and variable substitution, while single quotes treat variables as literals.
```
$name = "John";
$doubleQuoted = "Hello, $name!";
$singleQuoted = 'Hello, $name!';
```
Escaping characters and special characters in PHP quotes:
- Both single and double quotes can be escaped to include them in a string. Special characters like newline and tab can be represented using escape sequences.
```
$escapedSingle = 'This is a single-quoted string with \'escaped\' characters.';
$escapedDouble = "This is a double-quoted string with \"escaped\" characters.";
```
Handling literals and escape sequences in single and double quotes:
- Single quotes treat most characters as literals, while double quotes interpret escape sequences.
```
$literalSingle = 'This is a single-quoted string with \n literal newline.';
$escapedDouble = "This is a double-quoted string with\nescaped newline.";
```
Concatenation and expression evaluation in PHP quotes:
- Concatenation can be done using the dot (.) operator in both single and double quotes. Expression evaluation only occurs in double quotes.
```
$name = "John";
$concatenated = 'Hello, ' . $name . '!';
$evaluated = "2 + 2 = " . (2 + 2);
```
Impact on readability and maintainability with single and double quotes in PHP:
- Single quotes can improve readability by clearly indicating literals, while double quotes may enhance maintainability with variable interpolation.
```
$literalSingle = 'This is a single-quoted string with literal values.';
$interpolatedDouble = "This is a double-quoted string with $variable interpolation.";
```
Security considerations and preventing code injection with quotes in PHP:
- Use proper quoting to prevent code injection. Avoid directly embedding user input in double quotes without proper sanitization.
```
// Unsafe
$userInput = $_GET['input'];
$unsafeString = "User input: $userInput";

// Safe
$userInput = $_GET['input'];
$safeString = "User input: " . htmlspecialchars($userInput);
```