What's The Difference Between Single And Double Quotes In PHP?

Single Quoted Strings

Single quoted strings in PHP show content almost exactly as written. They interpret text literally, with minimal processing. Variables and most escape sequences are not evaluated within single quotes. There are two exceptions:

1. To include a single quote in a single quoted string, use a backslash: \'
2. To include a backslash, use two backslashes: \

Example:

$text = 'This is a single quoted string. It\'s simple and doesn\'t interpret variables like $var.';

Double Quoted Strings

Double quoted strings in PHP offer more options. They interpret escape sequences and evaluate variables within the string. This allows for dynamic content creation. Some features include:

1. Variable interpolation: "Hello, $name!"
2. Array access: "The first item is {$array[0]}"
3. Object property access: "The user's name is {$user->name}"

Double quoted strings also interpret escape sequences like \n for newline and \t for tab.

Heredoc Syntax

Heredoc provides another way to create strings, useful for multi-line content. It works similarly to double quoted strings. To use heredoc:

1. Start with !!! followed by an identifier
2. Write your string content on new lines
3. End with the same identifier

Example:

$text = !!!EOD
This is a heredoc string.
It can span multiple lines.
Variables like $var are interpreted.
EOD;

Heredoc is helpful for writing large blocks of HTML or SQL queries in PHP code.

Nowdoc Syntax

Nowdoc syntax, introduced in PHP 5.3.0, works like single quoted strings but for multi-line content. To use nowdoc:

1. Start with !!!'IDENTIFIER' (note the single quotes)
2. Write your string content on new lines
3. End with the same identifier

Example:

$text = !!!'EOD'
This is a nowdoc string.
It treats everything as literal text.
Variables like $var are not interpreted.
EOD;

Nowdoc is useful when you need to include example code or other content that shouldn't be processed by PHP.

Escaping Quotes Within Strings

When using strings in PHP, you might need to add quotes inside the string. The rules for escaping quotes depend on the string type:

For single quoted strings:

• To add a single quote, use a backslash before it: \'
• Double quotes don't need escaping

Example:

$singleQuoted = 'It\'s a single quoted string with "double quotes" inside.';

For double quoted strings:

• To add a double quote, use a backslash before it: \"
• Single quotes don't need escaping

Example:

$doubleQuoted = "This is a \"double quoted\" string with 'single quotes' inside.";

Performance Implications

Some people think single quoted strings are faster than double quoted strings in PHP. This is mostly incorrect:

1. Parsing speed: The speed difference between single and double quoted strings is very small in new PHP versions.

2. Opcode generation: PHP turns both string types into opcode during script compilation, so they run at the same speed.

3. Micro-optimization: Focusing on quote types for speed is often not needed. Writing clear, easy-to-maintain code is more important.

4. Variable interpolation: Double quotes can be faster when you need to put variables in strings, as you don't need to use concatenation.

A PHP core developer has said there's no big speed difference between single and double quotes. Simple tests often don't show the full picture of real-world scenarios and PHP's speed improvements.

When choosing between single and double quotes, focus on readability and what your string needs (like variable interpolation) instead of possible speed benefits.