Description
string 
str_pad ( string input, int pad_length [, string pad_string [, int pad_type]])
     This functions returns the input string 
     padded on the left, the right, or both sides to the specified
     padding length. If the optional argument
     pad_string is not supplied, the
     input is padded with spaces, otherwise it
     is padded with characters from pad_string
     up to the limit.
    
     Optional argument pad_type can be
     STR_PAD_RIGHT, STR_PAD_LEFT, or STR_PAD_BOTH.  If
     pad_type is not specified it is assumed to
     be STR_PAD_RIGHT.
    
     If the value of pad_length is negative or
     less than the length of the input string, no padding takes place.
    
     
例子 1. str_pad() example 
<?php $input = "Alien"; echo str_pad($input, 10);                      // produces "Alien     " echo str_pad($input, 10, "-=", STR_PAD_LEFT);  // produces "-=-=-Alien" echo str_pad($input, 10, "_", STR_PAD_BOTH);   // produces "__Alien___" echo str_pad($input, 6 , "___");               // produces "Alien_" ?>
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    注: 
      The pad_string may be truncated if the
      required number of padding characters can't be evenly divided by
      the pad_string's length.