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  1. <?php
  2.  
  3. $string = '99999999999999';
  4.  
  5. for($i=strlen($string); $i>0; $i-=4) $string=substr_replace($string, ' ', $i, 0);
  6.  
  7. echo $string;
Success #stdin #stdout 0s 82880KB
stdin
Standard input is empty
stdout
99 9999 9999 9999