var_dump(json_decode('[["10","荔枝好吃"],["11","葡萄好吃"],["18","菠萝好吃"]]'));

接下来怎么做你懂吧

去掉双引号不就好咯
$arr = trim($str,'"');
$res = $arr[0];

$arr = [["10","荔枝好吃"],["11","葡萄好吃"],["18","菠萝好吃"]];
echo "<pre>";
print_r(array_column($arr, 1, 0));

<?php

$array = [["10","荔枝好吃"],["11","葡萄好吃"],["18","菠萝好吃"]];
$data = array();
foreach ($array as $item) {
    $data[$item[0]] = $item[1];
}
echo '<pre>';
var_dump($data);

$str = '[["10","荔枝好吃"],["11","葡萄好吃"],["18","菠萝好吃"]]';// 字符串
$arr = array_reduce(json_decode($str,true),function($c,$v){$c[$v[0]]=$v[1];return $c;},[]);
var_dump($arr); // 对应数组
/*
array(3) {
  [10]=>
  string(12) "荔枝好吃"
  [11]=>
  string(12) "葡萄好吃"
  [18]=>
  string(12) "菠萝好吃"
}
*/