Most PHP coders will know that one is able to specify variable names in the following format, generally within double-quoted strings, however much they from on it:

$str = "Hello ${username}, you look nice today.";

However, that's not the only way to use the brace syntax!

Straying away from that common usage, it is perfectly reasonable to use the same format in a normal assignment operation:

// Same as $str = 'Hello Peter';
${'str'} = 'Hello Peter';

Now, moving into the bizzare, the string value within the curly braces can be any expression which results in a valid variable name (and more, see later)!

// Each are the same as $str = 'Hello Peter';
${'s'.'t'.'r'} = 'Hello Peter';
${substr('string', 0, 3)} = 'Hello Peter';
${ 'a' == 'b' ? 'rts' : 'str' } = 'Hello Peter';

Finally, lets get really crazy. The strings within the curly braces needn't necessarily strictly adhere to the normal variable naming conventions! Since the curly braces literally mean "evaluate my contents as a string", we can play around even more!

// You can't access these with 'normal' global variables, 
// but the curly brace form works as does getting at them 
// via the $GLOBALS superglobal variable!
// Values are converted to strings which is why 0xFF => '255', etc.
${007} = 'Hello Peter'; // $GLOBALS['7']
${0xFF} = 'Hello Peter'; // $GLOBALS['255']
${2 + 2} = 'Hello Peter'; // $GLOBALS['4']
${'Hello Peter'} = 'Hello Peter'; // $GLOBALS['Hello Peter']
${'cats, pet food, dogs'} = 'Hello Peter'; // $GLOBALS['cats, pet food, dogs']

I'm pretty sure these weird variable names serve absolutely no practical purpose, but it's interesting to know of little quirks like this (well, interesting for weirdos like me). Have a play around yourself, remembering that you can print_r() or var_dump() the superglobal $GLOBALS or even use get_defined_vars() to find out the variable names that have been created.