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<?php $in = '\u00E3\u0081\u0082a'; $out = preg_replace_callback('/\\\\u[0-9A-F]{4}/', function ($matches) { return hex2bin(substr($matches[0], -2, 2)); }, $in); echo $out;
Finding entry points
Branch analysis from position: 0
1 jumps found. (Code = 62) Position 1 = -2
filename:       /in/bSDdg
function name:  (null)
number of ops:  10
compiled vars:  !0 = $in, !1 = $out
line     #* E I O op                           fetch          ext  return  operands
-------------------------------------------------------------------------------------
   3     0  E >   ASSIGN                                                   !0, '%5Cu00E3%5Cu0081%5Cu0082a'
   4     1        INIT_FCALL                                               'preg_replace_callback'
         2        SEND_VAL                                                 '%2F%5C%5Cu%5B0-9A-F%5D%7B4%7D%2F'
         3        DECLARE_LAMBDA_FUNCTION                                  '%00%7Bclosure%7D%2Fin%2FbSDdg0x7f92097df099'
         4        SEND_VAL                                                 ~3
         5        SEND_VAR                                                 !0
         6        DO_ICALL                                         $4      
         7        ASSIGN                                                   !1, $4
   5     8        ECHO                                                     !1
   6     9      > RETURN                                                   1

Function %00%7Bclosure%7D%2Fin%2FbSDdg0x7f92097df099:
Finding entry points
Branch analysis from position: 0
1 jumps found. (Code = 62) Position 1 = -2
filename:       /in/bSDdg
function name:  {closure}
number of ops:  12
compiled vars:  !0 = $matches
line     #* E I O op                           fetch          ext  return  operands
-------------------------------------------------------------------------------------
   4     0  E >   RECV                                             !0      
         1        INIT_FCALL                                               'hex2bin'
         2        INIT_FCALL                                               'substr'
         3        FETCH_DIM_R                                      ~1      !0, 0
         4        SEND_VAL                                                 ~1
         5        SEND_VAL                                                 -2
         6        SEND_VAL                                                 2
         7        DO_ICALL                                         $2      
         8        SEND_VAR                                                 $2
         9        DO_ICALL                                         $3      
        10      > RETURN                                                   $3
        11*     > RETURN                                                   null

End of function %00%7Bclosure%7D%2Fin%2FbSDdg0x7f92097df099

Generated using Vulcan Logic Dumper, using php 7.3.0