interpolation: $result = "$hello $world";                  
  concatenation: $result = $hello.' '.$world;                
 reverse-concat: $result = $hello.(' '.$world);              
  interp-concat: $result = "$hello ".$world;                 
  concat-interp: $result = $hello." $world";                 
        sprintf: $result = sprintf("%s %s", $hello, $world); 
           join: $result = join(' ', $hello, $world);        
     super-join: $result = join('', $hello, ' ', $world);