would it make a difference if I omit single quotes in $_variable['variable']? [duplicate]

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Accessing arrays whitout quoting the key

I noticed there's a subtle difference... if I were to code this:

echo "Welcome, $_SESSION['username'], you are logged in.";

It will fail at parsing. However if I code like this:

echo "Welcome, $_SESSION[username], you are logged in.";

It works as expected which makes me wonder if single quotes are really necessary? I cannot find anything in PHP documentation showing that effect.

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In PHP, a global constant that isn't defined becomes a string.

Don't rely on this; always quote your array keys.

However, interpolated into a string, it is fine, as it is already a string.

Konforce makes a good point in the comments about using braces in string interpolation.

If you omit them, don't quote the key.

If you use them, you must quote the key, otherwise the constant will be looked up.

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This way is wrong but works$_SESSION[username] and take more time to parse the value of that associative index.

That effect PHP performance

Always use quotes around a string literal array index. For example, $foo['bar'] is correct, while $foo[bar] is not. This is wrong, but it works. The reason is that this code has an undefined constant (bar) rather than a string ('bar' - notice the quotes).PHP may in future define constants which, unfortunately for such code, have the same name. It works because PHP automatically converts a bare string (an unquoted string which does not correspond to any known symbol) into a string which contains the bare string. For instance, if there is no defined constant named bar, then PHP will substitute in the string 'bar' and use that.

you should use quotes while accessing values.

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Please check this document

in section Array do's and don'ts

<?php
// Show all errors
error_reporting(E_ALL);

$arr = array('fruit' => 'apple', 'veggie' => 'carrot');

// Correct
print $arr['fruit'];  // apple
print $arr['veggie']; // carrot

// Incorrect.  This works but also throws a PHP error of level E_NOTICE because
// of an undefined constant named fruit
// 
// Notice: Use of undefined constant fruit - assumed 'fruit' in...
print $arr[fruit];    // apple

// This defines a constant to demonstrate what's going on.  The value 'veggie'
// is assigned to a constant named fruit.
define('fruit', 'veggie');

// Notice the difference now
print $arr['fruit'];  // apple
print $arr[fruit];    // carrot

// The following is okay, as it's inside a string. Constants are not looked for
// within strings, so no E_NOTICE occurs here
print "Hello $arr[fruit]";      // Hello apple

// With one exception: braces surrounding arrays within strings allows constants
// to be interpreted
print "Hello {$arr[fruit]}";    // Hello carrot
print "Hello {$arr['fruit']}";  // Hello apple

// This will not work, and will result in a parse error, such as:
// Parse error: parse error, expecting T_STRING' or T_VARIABLE' or T_NUM_STRING'
// This of course applies to using superglobals in strings as well
print "Hello $arr['fruit']";
print "Hello $_GET['foo']";

// Concatenation is another option
print "Hello " . $arr['fruit']; // Hello apple
?>

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Inside a string you have to omit the single quotes or wrap the whole variable in {} ("...{$array['key']}..." or ...$array[key]...). However, wrapping it is highly recommended to prevent issues when having something like "...$foobar..." where you actually wanted "...{$foo}bar..." (i.e. the var $foo followed by bar).

But you might not want to use in-string vars at all but properly end the string: '...' . $var . '...'

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It's called bare strings as mentioned, strangly enough, in the array documentation. If no constant is found matching the bare string - It's, for historical reasons, assumbed to be a string literal. This syntax is however ridden with a lot of syntactic problems that I won't go into, also readability is a problem here. The reader questions himself - Is this a constant or a string?

Modern PHP versions emit a warning for this syntax, as to help fix this problem by using singly quoted strings ('username').

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yes.
If you pass an argument for an array without any quotes, php will first try to interpret the argument as a constant and if it isn't defined, it will act as expected.Even though it can give the same result, it is significantly slower that the quoted argument.
Here's an example of when this might not work :

define("a_constant","a value");
$a = array("a_constant"=>"the right value");
echo $a[a_constant]; 

The a_constant variable has the value "a value", so $a[a_constant] gets translated to $a["a value"], a key which does not exist in the array $a.