多次explode切割并组合，应该怎么做呢

$a = 'youku$$$tudou$$$down';$b='第一集$abcd第一集$abcd第二集$abcd第三集$abcd第四集$abcd第五集$abcd$$$第六集$abcd第七集$abcd第八集$ddddd第九集$abcd第十集$abcd第十一集$abcd$$$第十二集$abcd第十三集$abcd第十四集$abcd第十五集$abcd第十六集$abcd第十七集$efghijk';$a = explode('$$$', $a);$b = explode('$$$', $b);foreach($a as $i=>$r) {  echo "<li>$r</li>\n";  foreach(explode("\n", $b[$i]) as $v=>$lala) {    $t = explode('$', trim($v));    echo "<a href='".$i."_".$t[0].".html' target='_top'>$t[0]</a>\n";  }}

$a = explode('$$$', $a);$b = explode('$$$', $b);foreach($a as $i=>$r) {  echo "<li>$r</li>\n";  foreach(explode("\n", $b[$i]) as $j=>$v) {    $t = explode('$', trim($v));    printf("<a href='%d_%d.html' target='_top'>%s</a>\n", $i+1, $j+1, $t[0]);  }}

<li>youku</li><a href='1_1.html' target='_top'>第一集</a><a href='1_2.html' target='_top'>第一集</a><a href='1_3.html' target='_top'>第二集</a><a href='1_4.html' target='_top'>第三集</a><a href='1_5.html' target='_top'>第四集</a><a href='1_6.html' target='_top'>第五集</a><li>tudou</li><a href='2_1.html' target='_top'>第一集</a><a href='2_2.html' target='_top'>第一集</a><a href='2_3.html' target='_top'>第二集</a><a href='2_4.html' target='_top'>第三集</a><a href='2_5.html' target='_top'>第四集</a><a href='2_6.html' target='_top'>第五集</a><li>down</li><a href='3_1.html' target='_top'>第一集</a><a href='3_2.html' target='_top'>第一集</a><a href='3_3.html' target='_top'>第二集</a><a href='3_4.html' target='_top'>第三集</a><a href='3_5.html' target='_top'>第四集</a><a href='3_6.html' target='_top'>第五集</a>

$a = 'youku$$$tudou$$$down';$b='第一集$1_1.html第二集$1_2.html第三集$1_3.html第四集$1_3.html第五集$1_5.html$$$第一集$2_1.html第二集$2_2.html第三集$2_3.html第四集$2_4.html第五集$2_5.html$$$第一集$3_1.html第二集$3_2.html第三集$3_3.html第四集$3_4.html第五集$3_5.html'; $a = explode('$$$', $a);$b = explode('$$$', $b);foreach($a as $i=>$r) {  echo "<li>$r</li>\n";  foreach(explode("\n", $b[$i]) as $v) {    $t = explode('$', trim($v));    echo "<a href='$t[1]' target='_top'>$t[0]</a>\n";  }}

<li>youku</li><a href='1_1.html' target='_top'>第一集</a><a href='1_2.html' target='_top'>第二集</a><a href='1_3.html' target='_top'>第三集</a><a href='1_3.html' target='_top'>第四集</a><a href='1_5.html' target='_top'>第五集</a><li>tudou</li><a href='2_1.html' target='_top'>第一集</a><a href='2_2.html' target='_top'>第二集</a><a href='2_3.html' target='_top'>第三集</a><a href='2_4.html' target='_top'>第四集</a><a href='2_5.html' target='_top'>第五集</a><li>down</li><a href='3_1.html' target='_top'>第一集</a><a href='3_2.html' target='_top'>第二集</a><a href='3_3.html' target='_top'>第三集</a><a href='3_4.html' target='_top'>第四集</a><a href='3_5.html' target='_top'>第五集</a>