### Explanation

Let's analyze source code:

```
// https://www.php.net/manual/en/function.crc32.php#28012
function crc16($string) {
$crc = 0xFFFF;
for ($x = 0; $x < strlen ($string); $x++) {
$crc = $crc ^ ord($string[$x]);
for ($y = 0; $y < 8; $y++) {
if (($crc & 0x0001) == 0x0001) {
$crc = (($crc >> 1) ^ 0xA001);
} else { $crc = $crc >> 1; }
}
}
return $crc;
}

// https://stackoverflow.com/questions/507041/crc8-check-in-php/73305496#73305496
function crc8($input)
{
$crc8Table = [
0x00, 0x07, 0x0E, 0x09, 0x1C, 0x1B, 0x12, 0x15,
0x38, 0x3F, 0x36, 0x31, 0x24, 0x23, 0x2A, 0x2D,
0x70, 0x77, 0x7E, 0x79, 0x6C, 0x6B, 0x62, 0x65,
0x48, 0x4F, 0x46, 0x41, 0x54, 0x53, 0x5A, 0x5D,
0xE0, 0xE7, 0xEE, 0xE9, 0xFC, 0xFB, 0xF2, 0xF5,
0xD8, 0xDF, 0xD6, 0xD1, 0xC4, 0xC3, 0xCA, 0xCD,
0x90, 0x97, 0x9E, 0x99, 0x8C, 0x8B, 0x82, 0x85,
0xA8, 0xAF, 0xA6, 0xA1, 0xB4, 0xB3, 0xBA, 0xBD,
0xC7, 0xC0, 0xC9, 0xCE, 0xDB, 0xDC, 0xD5, 0xD2,
0xFF, 0xF8, 0xF1, 0xF6, 0xE3, 0xE4, 0xED, 0xEA,
0xB7, 0xB0, 0xB9, 0xBE, 0xAB, 0xAC, 0xA5, 0xA2,
0x8F, 0x88, 0x81, 0x86, 0x93, 0x94, 0x9D, 0x9A,
0x27, 0x20, 0x29, 0x2E, 0x3B, 0x3C, 0x35, 0x32,
0x1F, 0x18, 0x11, 0x16, 0x03, 0x04, 0x0D, 0x0A,
0x57, 0x50, 0x59, 0x5E, 0x4B, 0x4C, 0x45, 0x42,
0x6F, 0x68, 0x61, 0x66, 0x73, 0x74, 0x7D, 0x7A,
0x89, 0x8E, 0x87, 0x80, 0x95, 0x92, 0x9B, 0x9C,
0xB1, 0xB6, 0xBF, 0xB8, 0xAD, 0xAA, 0xA3, 0xA4,
0xF9, 0xFE, 0xF7, 0xF0, 0xE5, 0xE2, 0xEB, 0xEC,
0xC1, 0xC6, 0xCF, 0xC8, 0xDD, 0xDA, 0xD3, 0xD4,
0x69, 0x6E, 0x67, 0x60, 0x75, 0x72, 0x7B, 0x7C,
0x51, 0x56, 0x5F, 0x58, 0x4D, 0x4A, 0x43, 0x44,
0x19, 0x1E, 0x17, 0x10, 0x05, 0x02, 0x0B, 0x0C,
0x21, 0x26, 0x2F, 0x28, 0x3D, 0x3A, 0x33, 0x34,
0x4E, 0x49, 0x40, 0x47, 0x52, 0x55, 0x5C, 0x5B,
0x76, 0x71, 0x78, 0x7F, 0x6A, 0x6D, 0x64, 0x63,
0x3E, 0x39, 0x30, 0x37, 0x22, 0x25, 0x2C, 0x2B,
0x06, 0x01, 0x08, 0x0F, 0x1A, 0x1D, 0x14, 0x13,
0xAE, 0xA9, 0xA0, 0xA7, 0xB2, 0xB5, 0xBC, 0xBB,
0x96, 0x91, 0x98, 0x9F, 0x8A, 0x8D, 0x84, 0x83,
0xDE, 0xD9, 0xD0, 0xD7, 0xC2, 0xC5, 0xCC, 0xCB,
0xE6, 0xE1, 0xE8, 0xEF, 0xFA, 0xFD, 0xF4, 0xF3
];

$byteArray = unpack('C*', $input);
$len = count($byteArray);
$crc = 0;
for ($i = 1; $i <= $len; $i++) {
$crc = $crc8Table[($crc ^ $byteArray[$i]) & 0xff];
}
return $crc & 0xff;
}

$MYPASSWORD = "AdM1nP@assW0rd!";
include "flag.php";

if(isset($_POST['password']) && strlen($MYPASSWORD) == strlen($_POST['password'])) {
$pwhash1 = crc16($MYPASSWORD);
$pwhash2 = crc8($MYPASSWORD);

$password = $_POST['password'];
$pwhash3 = crc16($password);
$pwhash4 = crc8($password);

if($MYPASSWORD == $password) {
die("oops. Try harder!");
}
if($pwhash1 != $pwhash3) {
die("Oops. Nope. Try harder!");
}
if($pwhash2 != $pwhash4) {
die("OoOps. Not quite. Try harder!");
}
$access = true;

if($access) {
echo "You win a flag: $FLAG";
} else {
echo "Denied! :-(";
}
} else {
echo "Try harder!";
}
?>
```

As we can see, we have predefined password ```$MYPASSWORD = "AdM1nP@assW0rd!";```. We can't use it to login, because it's forbidden (and quite easy actually).
But, we can see ```crc16``` and ```crc8``` functions, which are used to compare hashes of our password from form and ```$MYPASSWORD```.

So, our goal to get a flag is create a password with the same ```crc16``` and ```crc8``` hashes.

Let's analyze our hash-functions deeper: ```crc8``` is default crc8 algorithm (that uses ```0x7``` polynomial as default). So, we can extract crc8 hash from predefined password with [this utility](https://www.sunshine2k.de/coding/javascript/crc/crc_js.html).

Next, ```crc16``` function. If we run provided PHP code with predefined password, we get ```0x61B2``` crc16 hash. Because of many variants of crc16 we should find polynomial value that is used in ```crc16``` function. Let's use [this utility](https://www.crccalc.com/?crc=AdM1nP@assW0rd!&method=CRC-16&datatype=ascii&outtype=hex). Let's look at the table and we can see that ```0x61B2``` value has the ```0x8005``` polynomial (CRC16-MODBUS).

And, final step: create a bruteforcer to create a password with 15 symbols that has hashes we found earlier.

```
import crcmod

def find_string_for_both_crc(target_crc16, target_crc8, max_length=15, charset="abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"):
crc16_func = crcmod.mkCrcFun(0x18005, initCrc=0xFFFF, rev=True, xorOut=0x0000)
crc8_func = crcmod.mkCrcFun(0x107, initCrc=0x00, rev=False, xorOut=0x00)

def recursive_search(prefix, depth):
if depth > max_length:
return None

current_crc16 = crc16_func(prefix.encode())
current_crc8 = crc8_func(prefix.encode())

if current_crc16 == target_crc16 and current_crc8 == target_crc8:
return prefix

for char in charset:
result = recursive_search(prefix + char, depth + 1)
if result:
return result
return None

return recursive_search("", 0)

target_crc16 = 0x61B2
target_crc8 = 0xA7

result = find_string_for_both_crc(target_crc16, target_crc8)
if result:
print(f"Found password: {result}")
else:
print("Password not found :(")
```

Bruteforcer found password: ```aaaaaaaaaac68LT```. Just use this value as password on website to get a flag! :)

### Flag

```ENO{Cr4hP_CRC_Collison_1N_P@ssw0rds!}```