-----
SQL
-----
---
CREATE TABLE `popot_tag` (
        `tag_id` int NOT NULL AUTO_INCREMENT,
        `tag_name` varchar(100) NOT NULL,
        PRIMARY KEY (`tag_id`),
        UNIQUE KEY `tag_name` (`tag_name`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
---
ALTER TABLE `popot_mod` ADD COLUMN `tag1_id` int NOT NULL AFTER mod_description;
ALTER TABLE `popot_mod` ADD COLUMN `tag2_id` int NOT NULL AFTER tag1_id;
ALTER TABLE `popot_mod` ADD COLUMN `tag3_id` int NOT NULL AFTER tag2_id;
---

-----
submit.php
-----
---
			(isset ($_POST['tag1_id'])) &&
			(isset ($_POST['tag2_id'])) &&
			(isset ($_POST['tag3_id'])) &&
---
			$iTag1ID = intval ($_POST['tag1_id']);
			$iTag2ID = intval ($_POST['tag2_id']);
			$iTag3ID = intval ($_POST['tag3_id']);
---
						tag1_id='" . $iTag1ID . "',
						tag2_id='" . $iTag2ID . "',
						tag3_id='" . $iTag3ID . "',
---
							tag1_id='" . $iTag1ID . "',
							tag2_id='" . $iTag2ID . "',
							tag3_id='" . $iTag3ID . "',
---

-----
popot_def.php
-----
---
function TagsDDL ($sIdName, $sPickText, $iActiveID)
/*****************************************************************************/
{
	/*** Returns the number of values, or FALSE. ***/

	$query_ddl = "SELECT
			tag_id,
			tag_name
		FROM `popot_tag`
		ORDER BY tag_name";
	$result_ddl = Query ($query_ddl);
	$iNrRows = mysqli_num_rows ($result_ddl);
	if ($iNrRows > 0)
	{
		print ('<select id="' . $sIdName . '" name="' . $sIdName . '">');
		print ('<option value="">' . $sPickText . '</option>');
		while ($row_ddl = mysqli_fetch_assoc ($result_ddl))
		{
			$iTagID = intval ($row_ddl['tag_id']);
			$sTagName = $row_ddl['tag_name'];

			print ('<option value="' . $iTagID . '"');
			if ($iTagID == $iActiveID) { print (' selected'); }
			print ('>' . Sanitize ($sTagName) . '</option>' . "\n");
		}
		print ('</select>');
		return ($iNrRows);
	} else {
		return (FALSE);
	}
}
/*****************************************************************************/
function TagLink ($iTagID)
/*****************************************************************************/
{
	/*** Returns a hyperlink, or FALSE. ***/

	if ($iTagID != 0)
	{
		$query_name = "SELECT
				tag_name
			FROM `popot_tag`
			WHERE (tag_id='" . $iTagID . "')";
		$result_name = Query ($query_name);
		if (mysqli_num_rows ($result_name) == 1)
		{
			$row_name = mysqli_fetch_assoc ($result_name);
			$sTagName = $row_name['tag_name'];
			return ('<a href="/custom_levels.php?action=Search&tag_id=' .
				$iTagID . '&pressed=Search">' . Sanitize ($sTagName) . '</a>');
		} else { return (FALSE); }
	} else { return (FALSE); }
}
/*****************************************************************************/
function TagMods ($iTagID)
/*****************************************************************************/
{
	$query_nr = "SELECT
			COUNT(*) AS nr
		FROM `popot_mod`
		WHERE (tag1_id='" . $iTagID . "')
		OR (tag2_id='" . $iTagID . "')
		OR (tag3_id='" . $iTagID . "')";
	$result_nr = Query ($query_nr);
	$row_nr = mysqli_fetch_assoc ($result_nr);
	return ($row_nr['nr']);
}
/*****************************************************************************/
---

-----
tags.php
-----
---
<?php
include_once (dirname (__FILE__) . '/popot_def.php');

/*****************************************************************************/
function ShowOverview ()
/*****************************************************************************/
{
	print ('<p>Three tags can be added to each mod by their authors, and <a href="/custom_levels.php?action=Search">mod search</a> allows filtering by tag. The available tags are limited to the controlled vocabulary listed alphabetically on this page. Tags must not contain spaces or more than two hyphenated words. You may <a href="/contact_faq.php">contact us</a> to suggest additional tags.</p>');

	$query_list = "SELECT
			tag_id
		FROM `popot_tag`
		ORDER BY tag_name";
	$result_list = Query ($query_list);
	$iNrRows = mysqli_num_rows ($result_list);
	if ($iNrRows > 0)
	{
		$iRow = 0;
		while ($row_list = mysqli_fetch_assoc ($result_list))
		{
			$iTagID = $row_list['tag_id'];

			print ("\n" . TagLink ($iTagID) . ' (' . TagMods ($iTagID) . ')');
			$iRow++;
			if ($iRow != $iNrRows) { print ('<br>'); }
		}
	} else {
		print ('No tags found.');
	}
}
/*****************************************************************************/
function ShowAdd ()
/*****************************************************************************/
{
print ('
<span style="display:block; text-align:center;">
<input type="text" name="tag_add" maxlength="100" placeholder="tag">
<input name="pressed" type="submit" value="Add">
</span>
');
}
/*****************************************************************************/
function ShowRemove ()
/*****************************************************************************/
{
	print ('<span style="display:block; margin-top:10px; text-align:center;">');
	if (TagsDDL ('tag_remove', 'Select...', 0) !== FALSE)
	{
		print ('<input name="pressed" type="submit" value="Remove"' .
			' style="margin-left:5px;">');
	} else {
		print ('No tags to remove.');
	}
	print ('</span>');
}
/*****************************************************************************/
function DoAdd ()
/*****************************************************************************/
{
	$sTag = $_POST['tag_add'];

	if ((strlen ($sTag) >= 1) && (strlen ($sTag) <= 100))
	{
		$query_exists = "SELECT
				tag_id
			FROM `popot_tag`
			WHERE (tag_name='" . mysqli_real_escape_string
				($GLOBALS['link'], $sTag) . "')";
		$result_exists = Query ($query_exists);
		if (mysqli_num_rows ($result_exists) == 0)
		{
			$query_add = "INSERT INTO `popot_tag` SET
				tag_name='" . mysqli_real_escape_string
					($GLOBALS['link'], $sTag) . "'";
			$result_add = Query ($query_add);
			if (mysqli_affected_rows ($GLOBALS['link']) == 1)
			{
				$GLOBALS['top_text'] = 'Added "' . Sanitize ($sTag) . '".';
				$GLOBALS['top_type'] = 'success';
			} else {
				$GLOBALS['top_text'] = 'Could not add "' . Sanitize ($sTag) . '".';
				$GLOBALS['top_type'] = 'error';
			}
		} else {
			$GLOBALS['top_text'] = 'Tag "' . Sanitize ($sTag) . '" already exists.';
			$GLOBALS['top_type'] = 'normal';
		}
	} else {
		$GLOBALS['top_text'] = 'Tag has an invalid size.';
		$GLOBALS['top_type'] = 'error';
	}
}
/*****************************************************************************/
function DoRemove ()
/*****************************************************************************/
{
	$iTagID = intval ($_POST['tag_remove']);

	if ($iTagID != 0)
	{
		$query_hits = "SELECT
				COUNT(*) AS hits
			FROM `popot_mod`
			WHERE (tag1_id='" . $iTagID . "')
			OR (tag2_id='" . $iTagID . "')
			OR (tag3_id='" . $iTagID . "')";
		$result_hits = Query ($query_hits);
		$row_hits = mysqli_fetch_assoc ($result_hits);
		if ($row_hits['hits'] == 0)
		{
			$query_remove = "DELETE FROM `popot_tag`
				WHERE (tag_id='" . $iTagID . "')";
			$result_remove = Query ($query_remove);
			if (mysqli_affected_rows ($GLOBALS['link']) == 1)
			{
				$GLOBALS['top_text'] = 'Removed tag.';
				$GLOBALS['top_type'] = 'success';
			} else {
				$GLOBALS['top_text'] = 'Could not remove tag.';
				$GLOBALS['top_type'] = 'error';
			}
		} else {
			$GLOBALS['top_text'] = 'Tag is in use.';
			$GLOBALS['top_type'] = 'error';
		}
	} else {
		$GLOBALS['top_text'] = 'No tag selected.';
		$GLOBALS['top_type'] = 'normal';
	}
}
/*****************************************************************************/

if ((isset ($_POST['pressed'])) && (IsAdmin() === TRUE))
{
	$sPressed = $_POST['pressed'];
	switch ($sPressed)
	{
		case 'Add': DoAdd(); break;
		case 'Remove': DoRemove(); break;
	}
}

StartHTML ('Custom Levels', 'Tags', 'custom_levels.php', 'Play');

if (IsAdmin() === FALSE)
{
	ShowOverview();
} else {
	AdminLinks();

	print ('<form name="input" action="tags.php" method="POST">');
	ShowAdd();
	ShowRemove();
	print ('</form>');
}

EndHTML();
?>
---

-----
custom_levels.php
-----
---
	if (isset ($_GET['tag_id']))
		{ $arSearch['tag_id'] = intval ($_GET['tag_id']); }
---
	if ($arSearch['tag_id'] != 0)
	{
		$sWhere = $sWhere . " AND ((tag1_id='" . $arSearch['tag_id'] . "') OR (tag2_id='" . $arSearch['tag_id'] . "') OR (tag3_id='" . $arSearch['tag_id'] . "'))";
	}
---
<tr>
<td>With Tag:</td>
<td colspan="2">
');
	TagsDDL ('tag_id', 'Any', $arSearch['tag_id']);
print ('
</td>
</tr>
---
	if (($row_get_mod['tag1_id'] != 0) || ($row_get_mod['tag2_id'] != 0) ||
		($row_get_mod['tag3_id'] != 0))
	{
		print ('Tags: ');
		$arTags = array();
		$sLink1 = TagLink ($row_get_mod['tag1_id']);
		if ($sLink1 !== FALSE) { array_push ($arTags, $sLink1); }
		$sLink2 = TagLink ($row_get_mod['tag2_id']);
		if ($sLink2 !== FALSE) { array_push ($arTags, $sLink2); }
		$sLink3 = TagLink ($row_get_mod['tag3_id']);
		if ($sLink3 !== FALSE) { array_push ($arTags, $sLink3); }
		print (implode (', ', $arTags));
		print ('<br>');
	}
---
			$arEdit['tag1_id'] = intval ($row_mod['tag1_id']);
			$arEdit['tag2_id'] = intval ($row_mod['tag2_id']);
			$arEdit['tag3_id'] = intval ($row_mod['tag3_id']);
---
		$arEdit['tag1_id'] = 0;
		$arEdit['tag2_id'] = 0;
		$arEdit['tag3_id'] = 0;
---
<h3>Tags</h3>

<p>Optionally, you may select up to three <a target="_blank" href="/tags.php">tags</a> below, that summarize your mod.</p>
');

	TagsDDL ('tag1_id', 'Select...', $arEdit['tag1_id']);
	TagsDDL ('tag2_id', 'Select...', $arEdit['tag2_id']);
	TagsDDL ('tag3_id', 'Select...', $arEdit['tag3_id']);

print ('
---
	form_data.append ("tag1_id", $("#tag1_id").val());
	form_data.append ("tag2_id", $("#tag2_id").val());
	form_data.append ("tag3_id", $("#tag3_id").val());
---
$arSearch['tag_id'] = 0;
---