$keywords_to_tags = $keywords_to_tags.',';
$keywords_to_tags = implode(',', array_unique(explode(',', $keywords_to_tags)));
$keywords_to_tags = explode(',', $keywords_to_tags);
$keywords_to_tags = array_flip($keywords_to_tags);

foreach ($keywords_to_tags as &$value) {
	if($value != '')
	{
	echo '<option>'$value.'</option>';
	}
}

$keywords_to_tags = 'a,b,c,a,b';
$tags = array_unique(explode(',', $keywords_to_tags));

foreach($tags as $tag) {
    echo "<option>{$tag}</option>", PHP_EOL;
}

<option>a</option>
<option>b</option>
<option>c</option>

$query_tags		= "
SELECT DISTINCT keywords FROM secao_enciclopedia
UNION ALL
SELECT DISTINCT keywords FROM secao_literatura
UNION ALL
SELECT DISTINCT keywords FROM secao_materias
UNION ALL
SELECT DISTINCT keywords as tabela FROM secao_noticias
";

$tags = mysqli_query($conexao, $sql);
$tags = array_unique(explode(',', join(',', $tags)));

$bancoDeDados = [
    'foo,bar',
    'bar',
    'giz,foo'
];

$tags = array_unique(explode(',', join(',', $bancoDeDados)));

foreach($tags as $tag) {
    echo "<option>{$tag}</option>", PHP_EOL;
}

<option>foo</option>
<option>bar</option>
<option>giz</option>

echo join(PHP_EOL, array_map(function ($tag) {
    return "<option>{$tag}</option>";
}, array_unique(explode(',', join(',', $bancoDeDados)))));

while($stmt_tags->fetch()) //INICIA LOOP
{
    //$bancoDeDados = ['foo,bar','bar','giz,foo'];
    $bancoDeDados = array($keywords_to_tags);
}

echo $keywords_to_tags);

while($stmt_tags->fetch()) //INICIA LOOP
{
echo $keywords_to_tags;
}