<?php
function glstrlen($str)
{
    $tmp_array = explode(",",$str);
    if(strlen($str)>30) {
        array_pop($tmp_array);
        $str = implode(",",$tmp_array);
        if(strlen($str)>30) {
            glstrlen($str);
        } else {
            return $str;
        }
    } else {
        return $str;
    }
}

$strs = "Woods,trees,grass ,grassland,Yellow, motorcycle,highway";
$a = glstrlen($strs);
echo $a;
?>

$strs = "Woods,trees,grass ,grassland,Yellow, motorcycle,highway";
$array = explode(',' , $strs);
print_r(array_pad($array,30,'.'));

------解决方案--------------------
array_pop：将数组最后一个单元弹出（出栈），返回的是弹出的单元，并不是弹出最后一个单元后的数组。

------解决方案--------------------
由于这是递归，所以你的return只是退出到上一层
PHP code
function glstrlen($str)
{
    $tmp_array = explode(",",$str);
    if(strlen($str)>30) {
        // var_dump($str);
        array_pop($tmp_array);
        $str = implode(",",$tmp_array);
        if(strlen($str)>30) {
            // 加了个判断，如果有返回值，那么就退出，这样层层退出，调用才算结束
            if(!is_null($str = glstrlen($str)) ) {
                return $str;
            }
        } else {
        
            return $str;
        }
    } else {
        return $str;
    }
   
}

------解决方案--------------------
PHP code
function glstrlen($str) {
    $arr =explode(',', $str);
    while(strlen($str) > 30) {
        array_pop($arr);
        $str = join(',', $arr);
    }
    return $str;
}
echo glstrlen("Woods,trees,grass ,grassland,Yellow, motorcycle,highway");

------解决方案--------------------
PHP code
function glstrlen($str) {
  if(strlen($str)>30) {
    $tmp_array = explode(",",$str);
    array_pop($tmp_array);
    $str = implode(",",$tmp_array);
    return glstrlen($str);
  }
  return $str;
}

$strs = "Woods,trees,grass ,grassland,Yellow, motorcycle,highway";
$a = glstrlen($strs);
echo $a;