(PHP 4 >= 4.2.0, PHP 5 <= 5.0.5, PHP 7)
dio_fcntl — Performs a c library fcntl on fd
   The dio_fcntl() function performs the
   operation specified by cmd on the file
   descriptor fd. Some commands require
   additional arguments args to be supplied.
  
fdThe file descriptor returned by dio_open().
cmdCan be one of the following operations:
          F_SETLK - Lock is set or cleared. If the lock
          is held by someone else dio_fcntl() returns
          -1.
         
          F_SETLKW - like F_SETLK,
          but in case the lock is held by someone else, 
          dio_fcntl() waits until the lock is released.
         
          F_GETLK - dio_fcntl() 
          returns an associative array (as described above) if someone else
          prevents lock. If there is no obstruction key "type" will set
          to F_UNLCK.
         
          F_DUPFD - finds the lowest numbered available
          file descriptor greater than or equal to args
          and returns them.
         
          F_SETFL - Sets the file descriptors flags to 
          the value specified by args, which can be
          O_APPEND, O_NONBLOCK or 
          O_ASYNC. To use O_ASYNC 
          you will need to use the PCNTL 
          extension.
         
args
       args is an associative array, when
       cmd is F_SETLK or
       F_SETLLW, with the following keys:
       
start - offset where lock begins
length - size of locked area. zero means to end of file
          whence - Where l_start is relative to: can be 
          SEEK_SET,
          SEEK_END and SEEK_CUR
         
          type - type of lock: can be
          F_RDLCK (read lock),
          F_WRLCK (write lock) or
          F_UNLCK (unlock)
         
Returns the result of the C call.
Example #1 Setting and clearing a lock
<?php
$fd = dio_open('/dev/ttyS0', O_RDWR);
if (dio_fcntl($fd, F_SETLK, Array("type"=>F_WRLCK)) == -1) {
   // the file descriptor appears locked
   echo "The lock can not be cleared. It is held by someone else.";
} else {
   echo "Lock successfully set/cleared";
}
dio_close($fd);
?>
Note: This function is not implemented on Windows platforms.