loop error in php code

Reason:- You are counting array length inside the loop and every time when any value got unset() from the array, length of array decreased and value of count($array) changed (simply decreased)

So logically your 5th and 6th element never goes through if condition (they never get traversed by loop because of decreasing length of the array )

Solution 1:- Put count outside and it will work properly:-

$count = count($arr);

//loop start from 0 so use < only otherwise, sometime you will get an undefined index error

    for ($i = 0; $i < $count; $i++) { 
        if ($arr[$i]{0} == '#') {
            //echo $arr[$i] . "\n";
            unset($arr[$i]);
        }
    }
    print_r($arr);

Output:-https://eval.in/996494

Solution 2:- That's why i prefer foreach() over for() loop

 foreach($arr as $key=> $ar){
        if ($ar[0] == '#') {
            unset($arr[$key]);
        }
    }
    print_r($arr); 

Output:-https://eval.in/996502

Try to use additional array to push right values. You calc count($arr); each iteration and when you do count($arr); your array gets smaller and count($arr); returns smaller values, so last elements won't be comparing, try to use variable to calc count before loop make changes:

<?php 
    //...
    $start_count = count($arr);
    for ($i = 0; $i <= $start_count; $i++) {
        if ($arr[$i]{0} == '#') {
            echo $arr[$i] . "\n";
            unset($arr[$i]);
        }
    }

Or remove bad element with a help of additional array, put good elements in new array and don't delete them from input array:

<?php

$arr = [
        '#EXTM3U',
        '#EXTINF:177,Paul Dateh & Oren Yoel - Be More',
        'Be More.mp3',
        '#EXTINF:291,Christopher Toy - Just Because',
        'Just Because.mp3',
        '#EXTINF:238,Magnetic North - Drift Away',
        'Drift Away.mp3'
    ];
    $cleared_from_mess_array = array();
    for ($i = 0; $i <= count($arr); $i++) {
        if ($arr[$i]{0} != '#') 
        {           
            array_push($cleared_from_mess_array,$arr[$i]);
        }
    }
    print_r($cleared_from_mess_array);

exit;