<?php
function days_in_month(string $month, $year): int
{
    return match(strtolower(substr($month, 0, 3))) {
        'jan' => 31,
        'feb' => is_leap($year) ? 29 : 28,
        'mar' => 31,
        'apr' => 30,
        'may' => 31,
        'jun' => 30,
        'jul' => 31,
        'aug' => 31,
        'sep' => 30,
        'oct' => 31,
        'nov' => 30,
        'dec' => 31,
        default => throw new InvalidArgumentException("Bogus month"),
    };
}
?>

can be more concisely written as

<?php
function days_in_month(string $month, $year): int
{
    return match(strtolower(substr($month, 0, 3))) {
        'apr', 'jun', 'sep',  'nov'  => 30,        
        'jan', 'mar', 'may', 'jul', 'aug',  'oct', 'dec'  => 31,
        'feb' => is_leap($year) ? 29 : 28,
        default => throw new InvalidArgumentException("Bogus month"),
    };
}
?>