Does PHP have a DEBUG symbol that can be used in code?

#DEFINE DEBUG
...

string returnedStr = this.SomeFoo();
#if DEBUG
    Debug.WriteLine("returned string =" + returnedStr);
#endif

define('DEBUG', true);
...
if (DEBUG):
  $debug->writeLine("stuff");
endif;

$str = 'string';
DEBUG ? $debug->writeLine("stuff is ".$str) : null;

define(DEBUG, true);

[...]

if(DEBUG) echo xdump::dump($debugOut);