How to output one of the elements of an array into a variable?

Question

PHP

Good evening, as a result of parsing a string, I get an array. How can I output the zero iteration to a variable so that I can write the result to the database?

$str = mysqli_real_escape_string($link, $str);
$pattern = '/(\d+)a/';
$res = preg_match($pattern, $str, $a);
$b = $a[0];