NBT.php

<?php

#Adding NBT tag "test" (string), which has a value of "hey"
/** @var Item $item */
$nbt = $item->getNamedTag() ?? new CompoundTag("", []);
$nbt->test = new StringTag("test", "hey");
$item->setNamedTag($nbt);

#If item has NBT tag "test", do something
/** @var Item $item */
if (isset($item->getNamedTag()->test)) {
    //$item has NBT tag: test.
    /** @var string $test */
    $test = $item->getNamedTag()->test;
}

#Unsetting/Removing NBT tag "test"
/** @var Item $item */
$nbt = $item->getNamedTag() ?? new CompoundTag("", []);
unset($nbt->test);
$item->setNamedTag($nbt);

#Adding NBT tag "test" (integer)
/** @var Item $item */
$nbt = $item->getNamedTag() ?? new CompoundTag("", []);
$nbt->test = new IntTag("test", 69);
$item->setNamedTag($nbt);

#If item has NBT tag "test", do something
/** @var Item $item */
if (isset($item->getNamedTag()->test)) {
    //$item has NBT tag: test.
    /** @var int $test */
    $test = $item->getNamedTag()->test->getValue();
}

#Unsetting/Removing NBT tag "test"
/** @var Item $item */
$nbt = $item->getNamedTag() ?? new CompoundTag("", []);
unset($nbt->test);
$item->setNamedTag($nbt);

#If the $item was in a (Player)$player's hand, you must update it after interfering with the NBT tag by...
$player->getInventory()->setItemInHand($item);

#For saving an array of integers to NBT, use IntArrayTag
/** @var Item $item */
$nbt = $item->getNamedTag() ?? new CompoundTag("", []);
$nbt->testarray = new IntArrayTag("testarray", [0, 8, 3, 1]);
$item->setNamedTag($nbt);

/** Checking if item has "testarray" tag. */
$nbt = $item->getNamedTag() ?? new CompoundTag("", []);
if (isset($nbt->testarray)) {
    //echo $nbt->testarray->getValue();
}

#You can also customize entity's and tile's NBT
/** @var Tile $tile */
$tile->namedtag->test = new StringTag("test", "hey");

/** @var Entity $entity */
$entity->namedtag->test = new StringTag("test", "hey");