gen_rand_string() don't return a string with the expected length

<?php

function unique_id()

{

	return bin2hex(random_bytes(8));

}

function gen_rand_string($num_chars = 8)

{

	// [a, z] + [0, 9] = 36

	return substr(strtoupper(base_convert(unique_id(), 16, 36)), 0, $num_chars);

}

for($i=0; $i<50; $i++)

    var_dump(gen_rand_string($i)).PHP_EOL;

string(0) ""

string(1) "T"

string(2) "CE"

string(3) "2VM"

string(4) "31RL"

string(5) "NDW2V"

string(6) "1TRJIT"

string(7) "2PQDO6F"

string(8) "1LPIANZF"

string(9) "1P2DJTWQ1"

string(10) "C9LEIBH8L6"

string(11) "14YVKR5UR8U"

string(12) "37NXW2SRIW8W"

string(12) "G8IZMLE151QJ"

string(11) "R3DX4D72URM"

string(13) "168OHOJFBZH8E"

string(13) "1S6EIO1OIO3OL"

string(12) "571VZIW74H2Q"

string(13) "17XQECQWRCOZ6"

string(13) "22L1OV0R9VR40"

string(13) "27BQ6LEIE98GW"

string(13) "24WC9I1YEGZOC"

string(12) "X7IRQORO19DZ"

string(13) "35H0EIEZUQ68G"

string(13) "209E289KQ7XCO"

string(12) "AL8N2QSTS3KI"

string(12) "TLUNEV4PX26M"

string(13) "3OR6H88SNX4W8"

string(12) "YUZTORZZ89FX"

string(13) "25QTZQM9RQV44"

string(13) "2OTEQ84PGA4GW"

string(13) "3J4XYMUZ2JACK"

string(13) "1WO2M7B65S1VR"

string(13) "3ELWZSUYE2W40"

string(13) "2D3G2LVIIQF4W"

string(12) "FKBVW28WXUCL"

string(12) "E6L7GO26JY3U"

string(13) "3FKQOEGEV6SK0"

string(12) "VW12L84722VG"

string(13) "158GCYLE9QCD6"

string(12) "YZLYSVWEEUBF"

string(12) "UOIF5K5F4MHR"

string(13) "2YI7TZ82UXWKG"

string(13) "2U63QAAN254W8"

string(13) "32B8OLL72IWWC"

string(13) "2K7L1L0E2Y804"

string(13) "2YW0X4KDU9C04"

string(13) "2YM148IHP7Y88"

string(13) "1WZCCIM5EOG2H"

string(12) "P7CS7KL5UU6R"