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.

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

It provides a way to embed a variable, an array value, or an object property in a string with a minimum of effort.

If a dollar sign ($) is encountered, the parser will greedily take as many tokens as possible to form a valid variable name. Enclose the variable name in curly braces to explicitly specify the end of the name.

<?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.";
// 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 apples.