What is the difference between single-quoted and double-quoted strings in PHP?

Question

I'm a little confused why I see some code in PHP with string placed in single quotes and sometimes in double quotes.

I just know in .NET, or the C language, if it is in a single quote, that means it is a character, not a string.

Answer 1

PHP strings can be specified not just in two ways, but in four ways.

1. Single quoted strings will display things almost completely "as is." Variables and most escape sequences will not be interpreted. The exception is that to display a literal single quote, you can escape it with a back slash \', and to display a back slash, you can escape it with another backslash \\ (So yes, even single quoted strings are parsed).
2. Double quote strings will display a host of escape sequences (including some regexes), and variables in the strings will be evaluated. An important point here is that you can use curly braces to isolate the name of the variable you want evaluated. For example let's say you have the variable $type and you want to echo "The $types are". That will look for the variable $types. To get around this use echo "The {$type}s are". Take a look at string parsing to see how to use array variables and such.
3. Heredoc string syntax works like double quoted strings. It starts with <<<. After this operator, an identifier is provided, then a newline. The string itself follows, and then the same identifier again to close the quotation. You don't need to escape quotes in this syntax.
4. Nowdoc (since PHP 5.3.0) string syntax works essentially like single quoted strings. The difference is that not even single quotes or backslashes have to be escaped. A nowdoc is identified with the same <<< sequence used for heredocs, but the identifier which follows is enclosed in single quotes, e.g. <<<'EOT'. No parsing is done in nowdoc.

Notes: Single quotes inside of single quotes and double quotes inside of double quotes must be escaped:

$string = 'He said "What\'s up?"';
$string = "He said \"What's up?\"";

Speed:
There is no difference.
Please read a credible article on the matter from one of PHP core developers. Speaking of tests, one should never take them for granted. It must be understood that writing a credible test and, especially, interpreting its results requires a lot of knowledge and experience. Which means that most tests out there are just bogus. For example, in a code like this

for($i=0;$i<100000;$i++) {
    'string';
}

The quoted string gets parsed only once, along with entire script, and then gets translated into opcode. Which is then gets executed a million times. So it measures anything but parsing. And that's only a tip of the iceberg. For a nanobenchmark like this, it's practically impossible to create a credible test that wouldn't be spoiled by some interfering side effect.

Answer 2

Things get evaluated in double quotes but not in single:

$s = "dollars";
echo 'This costs a lot of $s.'; // This costs a lot of $s.
echo "This costs a lot of $s."; // This costs a lot of dollars.

Answer 3

' Single quoted

The simplest way to specify a string is to enclose it in single quotes. Everything inside single quotes is treated as a plain string.

Example:

echo 'Start with a simple string';
echo 'Variables $var and escape sequences \n do not get interpolated';
echo 'You only need to escape the quote \' and a backslash itself\\';

---

" Double quoted

Strings in double quotes interpolate variables and a wide range of escape sequences. Note: Use curly braces {} to include complex variables or in case when next character after a variable is a valid character for a variable name.

Example:

echo "Start with a simple string";
echo "String's apostrophe";
echo "Escape sequences get interpolated\n";
echo "Variables $var get interpolated as well {$name}_sake";

---

Is there a performance benefit single quote vs double quote in PHP?

No.

In order to improve performance, PHP parses the entire code once and then stores the resulting bytecode in the opcode cache. This approach eliminates the entire parsing, along with whatever quoited strings as well.

Answer 4

A single-quoted string does not have variables within it interpreted. A double-quoted string does.

Also, a double-quoted string can contain apostrophes without backslashes, while a single-quoted string can contain unescaped quotation marks.

Answer 5

In PHP, both 'my name' and "my name" are string. You can read more about it at the PHP manual.

Thing you should know are

$a = 'name';
$b = "my $a"; == 'my name'
$c = 'my $a'; != 'my name'

In PHP, people use single quote to define a constant string, like 'a', 'my name', 'abc xyz', while using double quote to define a string contain identifier like "a $b $c $d".

Answer 6

Both kinds of enclosed characters are strings. One type of quote is conveniently used to enclose the other type of quote. "'" and '"'. The biggest difference between the types of quotes is that enclosed identifier references are substituted for inside double quotes, but not inside single quotes.

Answer 7

Some might say that I'm a little off-topic, but here it is anyway:

You don't necessarily have to choose because of your string's content between:
echo "It's \"game\" time."; or echo 'It\'s "game" time.';

If you're familiar with the use of the english quotation marks, and the correct character for the apostrophe, you can use either double or single quotes, because it won't matter anymore:
echo "It’s “game” time."; and echo 'It’s “game” time.';

Of course you can also add variables if needed. Just don't forget that they get evaluated only when in double quotes!