전체 코드입니다.

<?php
if (!defined('_GNUBOARD_')) exit; // 개별 페이지 접근 불가
if ($is_mobile) {
//ver1.0 150608 @_untitle_d

//글꼴 크기 px
define('G5_SC_MAXSIZE', 22); //maxsize
define('G5_SC_MINSIZE', 12); //minsize

define('G5_SC_POP_CNT', 20); //검색어 몇개
define('G5_SC_DATE_CNT', 7); //몇일 동안

define('G5_SC_STYLE', '<div style="margin:0 auto;width:100%;padding:3px;text-align:left;">');

function search_cloud()
{
    global $g5;
        
    $pop_array = array();
    $date_gap = date("Y-m-d", G5_SERVER_TIME - (60));
    
    $sql = "select pp_word, count(*) as cnt from {$g5['popular_table']}
                where pp_date between '$date_gap' and '".G5_TIME_YMD."'
                group by pp_word
                order by pp_id desc, pp_word
                limit 0, ".G5_SC_POP_CNT;
    $result = sql_query($sql);
    for ($i=0; $row=sql_fetch_array($result); $i++){
        $pop_array[$row[pp_word]] = $row[cnt];
    }
    
    return G5_SC_STYLE.tag_cloud($pop_array).'</div>';
}

//출처 https://youtu.be/iHxU6EAp6b4
function tag_cloud($tags)
{
    $maxsize = G5_SC_MAXSIZE;
    $minsize = G5_SC_MINSIZE;
    
    $maxval = max(array_values($tags));
    $minval = min(array_values($tags));
    
    $spread = ($maxval - $minval);
    $step = (($maxsize - $minsize) / $spread);
    
    $str = '';
    foreach($tags as $key => $value){
        $size = round($minsize + (($value - $minval) * $step));
        $str .= '<a href="'.G5_BBS_URL.'/search.php?sfl=wr_subject&sop=and&stx='.urlencode($key).'" style="font-size:'.$size.'px">'.$key.'</a> , ';
    }
    
    return $str;
}

} else {
//ver1.0 150608 @_untitle_d

//글꼴 크기 px
define('G5_SC_MAXSIZE', 22); //maxsize
define('G5_SC_MINSIZE', 12); //minsize

define('G5_SC_POP_CNT', 60); //검색어 몇개
define('G5_SC_DATE_CNT', 7); //몇일 동안

define('G5_SC_STYLE', '<div style="margin:0 auto;width:100%;padding:3px;text-align:left;">');

function search_cloud()
{
    global $g5;
        
    $pop_array = array();
    $date_gap = date("Y-m-d", G5_SERVER_TIME - (60));
    
    $sql = "select pp_word, count(*) as cnt from {$g5['popular_table']}
                where pp_date between '$date_gap' and '".G5_TIME_YMD."'
                group by pp_word
                order by pp_id desc, pp_word
                limit 0, ".G5_SC_POP_CNT;
    $result = sql_query($sql);
    for ($i=0; $row=sql_fetch_array($result); $i++){
        $pop_array[$row[pp_word]] = $row[cnt];
    }
    
    return G5_SC_STYLE.tag_cloud($pop_array).'</div>';
}

//출처 https://youtu.be/iHxU6EAp6b4
function tag_cloud($tags)
{
    $maxsize = G5_SC_MAXSIZE;
    $minsize = G5_SC_MINSIZE;
    
    $maxval = max(array_values($tags));
    $minval = min(array_values($tags));
    
    $spread = ($maxval - $minval);
    $step = (($maxsize - $minsize) / $spread);
    
    $str = '';
    foreach($tags as $key => $value){
        $size = round($minsize + (($value - $minval) * $step));
        $str .= '<a href="'.G5_BBS_URL.'/search.php?sfl=wr_subject&sop=and&stx='.urlencode($key).'" style="font-size:'.$size.'px">'.$key.'</a> , ';
    }
    
    return $str;
}

}
?>

오잉?

        $str .= '<a href="'.G5_BBS_URL.'/search.php?sfl=wr_subject&sop=and&stx='.urlencode($key).'" style="font-size:'.$size.'px">'.cut_str($key, 20).'</a> , ';

이렇게 하니까 되네요.