$arr = [
    '0' => '0',
    '2' => '2',
    '7_9'=> '7_9',
    '7_10'=> '7_10',
    '9' => '9'
];

$filtered = array_filter($arr, function ($key){
    return preg_match('#^7_#', $key);
}, ARRAY_FILTER_USE_KEY);


print_r($filtered);