Reverse reference of Regular Expression in preg_replace

Recently, I encountered a problem: to retrieve the SRC of all IMG in a log, and then regenerate the SRC Based on the SRC content, and use the preg_replace () function in PHP (), the reverse reference of a regular expression is used:

 $ Content     =     '  Text text ; other text ;  '  ;

  Function  Img_convert (  $ Img  , $ Param  ){
  $ Filepath     =  Getblogpicurl (  $ Img  );  //  Call getblogpicurl () to regenerate the new SRC value.  
    $ Param  =     Str_replace  ( "  \\  "  ,  $ Param  );
  $ Str     =  '  IMG src = "http://www.codebean.cn/wp-admin/  '  .  $ Filepath  .  '  "Alt =" "/> '  ;
}

  Function  Getblogpicurl (  $ Img  ){
  //  Re-generate the SRC valueCode  
  }

  $ Content    =     Preg_replace  ( "  // \/IE  "  ,  "  Img_convert ('$ 2',' $1 $3 ')  "  ,  $ Content  ); 

Here we use the reverse reference of the regular expression. when calling the img_convert () function, what is passed to img_convert () is the values of the regular expression matching three () respectively, they are in the format of $1, $2, $3, \ 1, \ 2, \ 3 respectively.

In addition, I directly use the img_convert () function in the replacement part of preg_replace, which requires adding the E parameter in the regular expression.