Thanks for your answer. This is my code.
$ops = [
"sora-hara",
"tuanbh",
"5000.000000 VESTS"
];
$hive = new Hive();
// create Obj privateKey
$key = $hive->privateKeyFrom(HIVE_DELEGATION_KEY);
// call API
$request = $hive->broadcast($key, 'delegate_vesting_shares', $ops);
It succeeds when VEST = 2000. AND >= 5000 then error.