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(PHP 4, PHP 5, PHP 7)
dba_exists — Check whether key exists
$key 
   ,  resource $handle 
   )  dba_exists()  checks whether the specified
   key exists in the database.
  
key The key the check is performed for.
handle The database handler, returned by dba_open() or dba_popen() .
   Returns  TRUE  if the key exists,  FALSE  otherwise.
  
[#1] frenzy dot ivan at gmail dot com [2008-11-26 17:37:37]
<?php
$index = 4;
$id = dba_open("frenzy.db", "w", "flatfile");
if(!$id) exit ("Some error...");
if(dba_exists($index, $id)) exit ("This key is using");
dba_close($id);
?>