What does ${ } mean in PHP syntax?

It's an embedded variable, so it knows where to stop looking for the end of the variable identifier.

${username} in a string means $username outside of a string. That way, it doesn't think $u is the variable identifier.

It's useful in cases like the URL that you gave, because then it doesn't need a space after the identifier.

See the php.net section about it.

The PHP documentation provides only a brief description of this usage, and I also have only seen it in a malware sample.

From the documentation:

In order to use variable variables with arrays, you have to resolve an ambiguity problem. That is, if you write $$a[1] then the parser needs to know if you meant to use $a[1] as a variable, or if you wanted $$a as the variable and then the [1] index from that variable. The syntax for resolving this ambiguity is: ${$a[1]} for the first case and ${$a}[1] for the second.

I read everywhere about using the ${var_name} and {$var_name} inside of strings in order to delimit variables, but I recently came across this:

<?php
$zb8b5 = 419;
$GLOBALS['t91a5'] = Array();
global $t91a5;
$t91a5 = $GLOBALS;
${"\x47\x4c\x4fB\x41\x4c\x53"}['t112f6f9'] = "\x63\x5c\x76\x48\x36\x47\x43\x7b\x35\x7c\x27...";
.
.
.

I found the above code when fixing a hacked website.

Note the last line. Turns out it is also possible to use the ${} syntax to declare variables with odd names.

So you can do (weird) things like:

<?php
${"my_var"} = 'asdf';
var_dump($my_var);
${"other_var"}['a_pos'] = 'my value';
var_dump($other_var);
?>

Output:

string(4) "asdf"
array(1) {
  ["a_pos"]=>
  string(8) "my value"
}

It's really a bad practice, of course, unless you're trying to scramble your code, as these guys wanted to do.

raw-bin hood pointed out a reference to the use of ${} outside strings in the PHP documentation: https://www.php.net/manual/en/language.variables.variable.php

${ } (dollar sign curly bracket) is known as Complex (curly) syntax.

This isn't called complex because the syntax is complex, but because it allows for the use of complex expressions.

Any scalar variable, array element or object property with a string representation can be included via this syntax. The expression is written the same way as it would appear outside the string, and then wrapped in { and }. Since { can not be escaped, this syntax will only be recognised when the $ immediately follows the {. Use {$ to get a literal {$. Some examples to make it clear:

<?php
$juice = "apple";

echo "He drank some $juice juice.".PHP_EOL;
// Invalid. "s" is a valid character for a variable name, but the variable is $juice.
echo "He drank some juice made of $juices.";
// Invalid
echo "He drank some juice made of { $juice}s.";
// Valid. Explicitly specify the end of the variable name by enclosing it in braces:
echo "He drank some juice made of ${juice}s.";
?>

The above example will output:

He drank some apple juice.
He drank some juice made of .
He drank some juice made of { apple}s.
He drank some juice made of apples.