utf8_encode and mb_convert_encoding Conflicting Resuts [duplicate]

I have a very annoying issue when transferring data from PHP (which is set up with default_charset = 'UTF-8') to MySQL, on an IIS environment with web.config set to:

<globalization requestEncoding="UTF-8" responseEncoding="UTF-8" />

Also tested on an Apache Environment with same default charset with same results. I also set MySQLi to set charset before any query:

$mysqli->set_charset('UTF-8');

I also have a set of prepared statements that search each character in a string to convert any none UTF-8 encoding to UTF-8 using:

mb_convert_encoding($char, 'UTF-8', mb_detect_encoding($char));

After trying and failing by parsing the whole string with the same process:

mb_convert_encoding($string, 'UTF-8', mb_detect_encoding($string));

The content type of the file with the script is set to content-type charset UTF-8 on the header. And still none of this has fixed the issue.

I also, have set all collations of the MySQL tables and schemas to utf8mb4, and of course using `utf8mb4_unicode_520_ci'. Still, issue persists.

The function I wrote to split the string is as follows:

    static private function split_and_convert($value, $encoding = 'UTF-8') {
        $split = mb_str_split($value);
        $filter = [];
        foreach($split as $chr) {
            $from = mb_detect_encoding($chr);
            $encoded = ($from !== $encoding)
                ? mb_convert_encoding($chr, $encoding, $from)
                : $chr;
            $filter[] = $encoded;
            // echoing for testing
            echo 'char ' . $encoded . ' (' . $from . ')<br>';
        }
        return join('', $filter);
    }

Which echo the following trying to save Martínez:

char M (ASCII)
char a (ASCII)
char r (ASCII)
char t (ASCII)
char � (UTF-8)
char n (ASCII)
char e (ASCII)
char z (ASCII)

With MySQL throwing the following error:

Error No: 1366 - Incorrect string value: '\xEDnez' for column 'contactLast' at row 1

Still no joy. But if I do this, it works like a charm:

    static private function split_and_convert($value, $encoding = 'UTF-8') {
        $split = mb_str_split($value);
        $filter = [];
        foreach($split as $chr) {
            $from = mb_detect_encoding($chr);
            $encoded = ($from !== $encoding)
                ? mb_convert_encoding($chr, $encoding, $from)
                // set to actually encode the 'UTF-8' encoded char
                // I'm supposed not to do this, but is what works
                // which makes no sense
                : utf8_encode($chr);
            $filter[] = $encoded;
            // echoing for testing
            echo 'char ' . $encoded . ' (' . $from . ')<br>';
        }
        return join('', $filter);
    }

With the following results:

char M (ASCII)
char a (ASCII)
char r (ASCII)
char t (ASCII)
char í (UTF-8)
char n (ASCII)
char e (ASCII)
char z (ASCII)

And MySQL not throwing any error. If I replace the utf8_encode with mb_convert_encoding($encoded, 'UTF-8'), I get the same errors as not filtering the string.

I'm suspecting mb_detect_encoding is either detecting erroneously the encoding of the characters or simply the mb_convert_encoding is not doing it's job.

If anyone can help I will greatly appreciate it. I'm been searching for hours for a solution and none of the solutions offered have worked. And I find myself in a pickle because what works has been deprecated.

So, these are the list of the issues I'm facing:

1. mb_convert_encoding -> not encoding or encoding to something other than UTF8
2. mb_detect_encoding -> not detecting correctly
3. Strings is encoded as ASCII not UTF-8 as charset in header requested.
4. Confusion: Why I need to encode to UTF-8 an UTF-8 encoded character to work?