The following
Warning: Undefined variable $concerts in /home/cybertester/public_html/2223/PHPTEXTBOOK/Ch1/Concerts.php on line 20
Fatal error: Uncaught TypeError: count(): Argument #1 ($value) must be of type Countable|array, null given in /home/cybertester/public_html/2223/PHPTEXTBOOK/Ch1/Concerts.php:20
Stack trace:
#0 {main}
thrown in /home/cybertester/public_html/2223/PHPTEXTBOOK/Ch1/Concerts.php on line 20