Comma separated string to parent child relationship array php

Try this simply making own function as

$str = "word1,word2,word3";
$res = [];

function makeNested($arr) {
    if(count($arr)<2)
        return $arr;
    $key = array_shift($arr);
    return array($key => makeNested($arr));
}

print_r(makeNested(explode(',', $str)));

Demo

function tooLazyToCode($string)
{
    $structure = null;
    foreach (array_reverse(explode(',', $string)) as $part) {
        $structure = ($structure == null) ? $part : array($part => $structure);
    }

    return $structure;
}

Please check below code it will take half of the time of the above answers:

<?php
$str = "sports,cricket,football,hockey,tennis";
$arr = explode(',', $str);
$result = array();
$arr_len = count($arr) - 1;
$prev = $arr_len;
for($i = $arr_len; $i>=0;$i--){
    if($prev != $i){
        $result = array($arr[$i] => $result);
    } else {
        $result = array ($arr[$i]);
    }
    $prev = $i;
}
echo '<pre>',print_r($result),'</pre>';

Here is another code for you, it will give you result as you have asked :

<?php
$str = "sports,cricket,football,hockey,tennis";
$arr = explode(',', $str);
$result = array();
$arr_len = count($arr) - 1;
$prev = $arr_len;
for($i = $arr_len; $i>=0;$i--){
    if($prev != $i){
        if($i == 0){
            $result = array($arr[$i] => $result);
        }else{
            $result = array(array($arr[$i] => $result));
        }

    } else {
        $result = array ($arr[$i]);
    }
    $prev = $i;
}
echo '<pre>',print_r($result),'</pre>';