Description:
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The functions "dba_firstkey" & "dba_nextkey" retuns NULL (instead of false) on invalid dba resource.
It's documented as "Returns the key on success or FALSE on failure."
That can result in an infinite loop on iterate over items.

Test script:
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$dba = false;

var_dump(dba_firstkey($dba));
var_dump(dba_nextkey($dba));

Expected result:
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PHP Warning:  dba_firstkey() expects parameter 1 to be resource, boolean given in /tmp/dba_firstkey_nextkey.php on line 5

Warning: dba_firstkey() expects parameter 1 to be resource, boolean given in /tmp/dba_firstkey_nextkey.php on line 5
boolean(false)
PHP Warning:  dba_nextkey() expects parameter 1 to be resource, boolean given in /tmp/dba_firstkey_nextkey.php on line 6

Warning: dba_nextkey() expects parameter 1 to be resource, boolean given in /tmp/dba_firstkey_nextkey.php on line 6
boolean(false)

Actual result:
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PHP Warning:  dba_firstkey() expects parameter 1 to be resource, boolean given in /tmp/dba_firstkey_nextkey.php on line 5

Warning: dba_firstkey() expects parameter 1 to be resource, boolean given in /tmp/dba_firstkey_nextkey.php on line 5
NULL
PHP Warning:  dba_nextkey() expects parameter 1 to be resource, boolean given in /tmp/dba_firstkey_nextkey.php on line 6

Warning: dba_nextkey() expects parameter 1 to be resource, boolean given in /tmp/dba_firstkey_nextkey.php on line 6
NULL