$s = '12_1_5_10';

$r[] = strtok($s, '_');
for($i=0; $t = strtok('_'); $i++) {
  $r[] = $r[$i] . '_' . $t;
}
print_r($r);

------解决方案--------------------
还是老大猛，我的就是裹脚布

$str = '12_1_5_10';
$array = explode('_', $str);
$newarr = array();
for ($i = 0; $i < count($array); $i++) {
   $newarr[]=array_slice($array,0,$i+1);
}
foreach($newarr as $value){
   echo '<pre>'.implode('_',$value).'</pre>';
}