PHP中$a=1;$b=&$a;$c=(++$a)+(++$a)+(++$a); $c为啥等于10?

<code class="text">line     #* E I O op                           fetch          ext  return  operands
-------------------------------------------------------------------------------------
   3     0  E >   ASSIGN                                                   !0, 1
   4     1        ASSIGN_REF                                               !1, !0
   5     2        PRE_INC                                          $2      !0
         3        PRE_INC                                          $3      !0
         4        ADD                                              ~4      $2, $3
         5        PRE_INC                                          $5      !0
         6        ADD                                              ~6      ~4, $5
         7        ASSIGN                                                   !2, ~6
   6     8        SEND_VAR                                                 !2
         9        DO_FCALL                                      1          'var_dump'
        10      > RETURN                                                   1
</code>