Description
void 
gmp_setbit ( resource &a, int index [, bool set_clear])
     Sets bit index in
     a. set_clear
     defines if the bit is set to 0 or 1. By default the bit is set to
     1. Index starts at 0.
    
注: 
      Unlike most of the other GMP functions, gmp_setbit()
      must be called with a GMP resource that already exists (using
      gmp_init() for example). One will not be
      automatically created.
     
例子 1. gmp_setbit() example 
<?php $a = gmp_init("0xfd"); gmp_setbit($a, 1); // index starts at 0 echo gmp_strval($a) . "\n"; ?>
 |  
 
      The printout of the above program will be:
       | 
     See also gmp_clrbit().