you have to return the integer n such as
𝑛
3

(
𝑛
−
1
)
3
+
(
𝑛
−
2
)
3
+
.
.
.
+
1
3

𝑚
n
3
+(n−1)
3
+(n−2)
3
+...+1
3
=m if such a n exists or -1 if there is no such n.

OP has implemented the logic, it is basically summation of cubes of numbers from 1 to ... until it reaches or surpasses $m

I'm looking atht explination and just don't understand how it works. I'm going to have to study the wiki for this one as well.

I agree. I think test cases doesn't cover all. In my first solution I skip lower case everything except first character and I pass it.

Really like this one! It's have a perfect balance between complexity and simplicity. My only suggestion is maybe make description is more explicit? I was need to read the Wikipedia before start because I don't understand how cypher work. Or maybe it's just me.

I think this solution is fit perfect for given description. The only one thing which I would correct is implode($arr). There is no need in delimiter at all.

str_split() is one which I need to remember. Doesn't use it and create arrays from strings in a strange manner.

Doesn't know that first parameter is optional, thank to you will be use it in future :)

I know there must be function for that but I can't find it in PHP manual and don't want to just google it. Thank you for sharing your knowlendge!