A while back I wondered the same thing, and ended up benchmarking a number of approaches:

  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);
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The results, fastest first:

reverse-concat: 14.24
 interp-concat: 14.43
 concat-interp: 14.68
 interpolation: 14.98
 concatenation: 15.74
    super-join: 17.65
          join: 17.68
       sprintf: 24.65
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And I concluded that I would never use sprintf for this, and there really wasn't much difference between the other approaches, so I should just focus on what was easiest for someone to understand in any given context.