Import global functions and constants for enhanced performance

This is better for performance because these then don't need to be reevaluated every time they are called.

When encountering an unqualified function or constant reference, PHP will first try to locate a symbol in the current namespace by that name, and then fall back to the global namespace.
This short-circuits the check, which has substantial performance effects in some cases - in particular, ord(), chr() and strlen() show ~1500x faster calls when they are fully qualified.

However, this doesn't mean that PM is getting a massive amount faster. In real world terms, this translates to about 10-15% performance improvement.
But before anyone gets excited, you should know that the CodeOptimizer in the PreProcessor repo has been applying fully-qualified symbol optimizations to Jenkins builds for years, which is one of the reasons why Jenkins builds have better performance than home-built or source installations.
We're choosing to do this for the sake of future SafePHP integration and also to be able to get rid of the buggy CodeOptimizer, so that phar and source are more consistent.

src/pocketmine/network/mcpe/protocol/AddEntityPacket.php

@@ -30,6 +30,8 @@ use pocketmine\entity\EntityIds;
 use pocketmine\math\Vector3;
 use pocketmine\network\mcpe\NetworkSession;
 use pocketmine\network\mcpe\protocol\types\EntityLink;
+use function array_search;
+use function count;
 
 class AddEntityPacket extends DataPacket{
 	public const NETWORK_ID = ProtocolInfo::ADD_ENTITY_PACKET;

src/pocketmine/network/mcpe/protocol/AddPlayerPacket.php

@@ -30,6 +30,7 @@ use pocketmine\math\Vector3;
 use pocketmine\network\mcpe\NetworkSession;
 use pocketmine\network\mcpe\protocol\types\EntityLink;
 use pocketmine\utils\UUID;
+use function count;
 
 class AddPlayerPacket extends DataPacket{
 	public const NETWORK_ID = ProtocolInfo::ADD_PLAYER_PACKET;

src/pocketmine/network/mcpe/protocol/AvailableCommandsPacket.php

@@ -29,6 +29,13 @@ use pocketmine\network\mcpe\NetworkSession;
 use pocketmine\network\mcpe\protocol\types\CommandData;
 use pocketmine\network\mcpe\protocol\types\CommandEnum;
 use pocketmine\network\mcpe\protocol\types\CommandParameter;
+use function array_flip;
+use function array_keys;
+use function array_map;
+use function array_search;
+use function array_values;
+use function count;
+use function dechex;
 
 class AvailableCommandsPacket extends DataPacket{
 	public const NETWORK_ID = ProtocolInfo::AVAILABLE_COMMANDS_PACKET;
@@ -346,13 +353,13 @@ class AvailableCommandsPacket extends DataPacket{
 			}
 		}
 
-		$this->enumValues = array_map('strval', array_keys($enumValuesMap)); //stupid PHP key casting D:
+		$this->enumValues = array_map('\strval', array_keys($enumValuesMap)); //stupid PHP key casting D:
 		$this->putUnsignedVarInt($this->enumValuesCount = count($this->enumValues));
 		foreach($this->enumValues as $enumValue){
 			$this->putString($enumValue);
 		}
 
-		$this->postfixes = array_map('strval', array_keys($postfixesMap));
+		$this->postfixes = array_map('\strval', array_keys($postfixesMap));
 		$this->putUnsignedVarInt(count($this->postfixes));
 		foreach($this->postfixes as $postfix){
 			$this->putString($postfix);

src/pocketmine/network/mcpe/protocol/AvailableEntityIdentifiersPacket.php

@@ -26,6 +26,7 @@ namespace pocketmine\network\mcpe\protocol;
 #include <rules/DataPacket.h>
 
 use pocketmine\network\mcpe\NetworkSession;
+use function base64_decode;
 
 class AvailableEntityIdentifiersPacket extends DataPacket{
 	public const NETWORK_ID = ProtocolInfo::AVAILABLE_ENTITY_IDENTIFIERS_PACKET;

src/pocketmine/network/mcpe/protocol/BatchPacket.php

@@ -28,6 +28,11 @@ namespace pocketmine\network\mcpe\protocol;
 
 use pocketmine\network\mcpe\NetworkBinaryStream;
 use pocketmine\network\mcpe\NetworkSession;
+use function assert;
+use function get_class;
+use function strlen;
+use function zlib_decode;
+use function zlib_encode;
 #ifndef COMPILE
 use pocketmine\utils\Binary;
 #endif

src/pocketmine/network/mcpe/protocol/ClientboundMapItemDataPacket.php

@@ -31,6 +31,8 @@ use pocketmine\network\mcpe\NetworkSession;
 use pocketmine\network\mcpe\protocol\types\DimensionIds;
 use pocketmine\network\mcpe\protocol\types\MapTrackedObject;
 use pocketmine\utils\Color;
+use function assert;
+use function count;
 
 class ClientboundMapItemDataPacket extends DataPacket{
 	public const NETWORK_ID = ProtocolInfo::CLIENTBOUND_MAP_ITEM_DATA_PACKET;

src/pocketmine/network/mcpe/protocol/CommandOutputPacket.php

@@ -28,6 +28,7 @@ namespace pocketmine\network\mcpe\protocol;
 use pocketmine\network\mcpe\NetworkSession;
 use pocketmine\network\mcpe\protocol\types\CommandOriginData;
 use pocketmine\network\mcpe\protocol\types\CommandOutputMessage;
+use function count;
 
 class CommandOutputPacket extends DataPacket{
 	public const NETWORK_ID = ProtocolInfo::COMMAND_OUTPUT_PACKET;

src/pocketmine/network/mcpe/protocol/CraftingDataPacket.php

@@ -32,6 +32,8 @@ use pocketmine\inventory\ShapelessRecipe;
 use pocketmine\item\Item;
 use pocketmine\network\mcpe\NetworkBinaryStream;
 use pocketmine\network\mcpe\NetworkSession;
+use function count;
+use function str_repeat;
 
 class CraftingDataPacket extends DataPacket{
 	public const NETWORK_ID = ProtocolInfo::CRAFTING_DATA_PACKET;

src/pocketmine/network/mcpe/protocol/CraftingEventPacket.php

@@ -28,6 +28,7 @@ namespace pocketmine\network\mcpe\protocol;
 use pocketmine\item\Item;
 use pocketmine\network\mcpe\NetworkSession;
 use pocketmine\utils\UUID;
+use function count;
 
 class CraftingEventPacket extends DataPacket{
 	public const NETWORK_ID = ProtocolInfo::CRAFTING_EVENT_PACKET;

src/pocketmine/network/mcpe/protocol/DataPacket.php

@@ -29,6 +29,11 @@ use pocketmine\network\mcpe\CachedEncapsulatedPacket;
 use pocketmine\network\mcpe\NetworkBinaryStream;
 use pocketmine\network\mcpe\NetworkSession;
 use pocketmine\utils\Utils;
+use function bin2hex;
+use function get_class;
+use function is_object;
+use function is_string;
+use function method_exists;
 
 abstract class DataPacket extends NetworkBinaryStream{
 

src/pocketmine/network/mcpe/protocol/ExplodePacket.php

@@ -28,6 +28,7 @@ namespace pocketmine\network\mcpe\protocol;
 
 use pocketmine\math\Vector3;
 use pocketmine\network\mcpe\NetworkSession;
+use function count;
 
 class ExplodePacket extends DataPacket{
 	public const NETWORK_ID = ProtocolInfo::EXPLODE_PACKET;

src/pocketmine/network/mcpe/protocol/InventoryContentPacket.php

@@ -27,6 +27,7 @@ namespace pocketmine\network\mcpe\protocol;
 
 use pocketmine\item\Item;
 use pocketmine\network\mcpe\NetworkSession;
+use function count;
 
 class InventoryContentPacket extends DataPacket{
 	public const NETWORK_ID = ProtocolInfo::INVENTORY_CONTENT_PACKET;

src/pocketmine/network/mcpe/protocol/InventoryTransactionPacket.php

@@ -27,6 +27,7 @@ namespace pocketmine\network\mcpe\protocol;
 
 use pocketmine\network\mcpe\NetworkSession;
 use pocketmine\network\mcpe\protocol\types\NetworkInventoryAction;
+use function count;
 
 class InventoryTransactionPacket extends DataPacket{
 	public const NETWORK_ID = ProtocolInfo::INVENTORY_TRANSACTION_PACKET;

src/pocketmine/network/mcpe/protocol/LoginPacket.php

@@ -30,6 +30,8 @@ use pocketmine\network\mcpe\NetworkSession;
 use pocketmine\utils\BinaryStream;
 use pocketmine\utils\MainLogger;
 use pocketmine\utils\Utils;
+use function get_class;
+use function json_decode;
 
 class LoginPacket extends DataPacket{
 	public const NETWORK_ID = ProtocolInfo::LOGIN_PACKET;

src/pocketmine/network/mcpe/protocol/PacketPool.php

@@ -23,6 +23,8 @@ declare(strict_types=1);
 
 namespace pocketmine\network\mcpe\protocol;
 
+use function ord;
+
 class PacketPool{
 	/** @var \SplFixedArray<DataPacket> */
 	protected static $pool = null;

src/pocketmine/network/mcpe/protocol/PlayerListPacket.php

@@ -29,6 +29,7 @@ namespace pocketmine\network\mcpe\protocol;
 use pocketmine\entity\Skin;
 use pocketmine\network\mcpe\NetworkSession;
 use pocketmine\network\mcpe\protocol\types\PlayerListEntry;
+use function count;
 
 class PlayerListPacket extends DataPacket{
 	public const NETWORK_ID = ProtocolInfo::PLAYER_LIST_PACKET;

src/pocketmine/network/mcpe/protocol/PurchaseReceiptPacket.php

@@ -26,6 +26,7 @@ namespace pocketmine\network\mcpe\protocol;
 #include <rules/DataPacket.h>
 
 use pocketmine\network\mcpe\NetworkSession;
+use function count;
 
 class PurchaseReceiptPacket extends DataPacket{
 	public const NETWORK_ID = ProtocolInfo::PURCHASE_RECEIPT_PACKET;

src/pocketmine/network/mcpe/protocol/ResourcePackChunkDataPacket.php

@@ -28,6 +28,7 @@ namespace pocketmine\network\mcpe\protocol;
 
 
 use pocketmine\network\mcpe\NetworkSession;
+use function strlen;
 
 class ResourcePackChunkDataPacket extends DataPacket{
 	public const NETWORK_ID = ProtocolInfo::RESOURCE_PACK_CHUNK_DATA_PACKET;

src/pocketmine/network/mcpe/protocol/ResourcePackClientResponsePacket.php

@@ -27,6 +27,7 @@ namespace pocketmine\network\mcpe\protocol;
 
 
 use pocketmine\network\mcpe\NetworkSession;
+use function count;
 
 class ResourcePackClientResponsePacket extends DataPacket{
 	public const NETWORK_ID = ProtocolInfo::RESOURCE_PACK_CLIENT_RESPONSE_PACKET;

src/pocketmine/network/mcpe/protocol/ResourcePackStackPacket.php

@@ -29,6 +29,7 @@ namespace pocketmine\network\mcpe\protocol;
 
 use pocketmine\network\mcpe\NetworkSession;
 use pocketmine\resourcepacks\ResourcePack;
+use function count;
 
 class ResourcePackStackPacket extends DataPacket{
 	public const NETWORK_ID = ProtocolInfo::RESOURCE_PACK_STACK_PACKET;

src/pocketmine/network/mcpe/protocol/ResourcePacksInfoPacket.php

@@ -28,6 +28,7 @@ namespace pocketmine\network\mcpe\protocol;
 
 use pocketmine\network\mcpe\NetworkSession;
 use pocketmine\resourcepacks\ResourcePack;
+use function count;
 
 class ResourcePacksInfoPacket extends DataPacket{
 	public const NETWORK_ID = ProtocolInfo::RESOURCE_PACKS_INFO_PACKET;

src/pocketmine/network/mcpe/protocol/SetScorePacket.php

@@ -27,6 +27,7 @@ namespace pocketmine\network\mcpe\protocol;
 
 use pocketmine\network\mcpe\NetworkSession;
 use pocketmine\network\mcpe\protocol\types\ScorePacketEntry;
+use function count;
 
 class SetScorePacket extends DataPacket{
 	public const NETWORK_ID = ProtocolInfo::SET_SCORE_PACKET;

src/pocketmine/network/mcpe/protocol/SetScoreboardIdentityPacket.php

@@ -27,6 +27,7 @@ namespace pocketmine\network\mcpe\protocol;
 
 use pocketmine\network\mcpe\NetworkSession;
 use pocketmine\network\mcpe\protocol\types\ScoreboardIdentityPacketEntry;
+use function count;
 
 class SetScoreboardIdentityPacket extends DataPacket{
 	public const NETWORK_ID = ProtocolInfo::SET_SCOREBOARD_IDENTITY_PACKET;

src/pocketmine/network/mcpe/protocol/StartGamePacket.php

@@ -30,6 +30,9 @@ use pocketmine\math\Vector3;
 use pocketmine\network\mcpe\NetworkBinaryStream;
 use pocketmine\network\mcpe\NetworkSession;
 use pocketmine\network\mcpe\protocol\types\PlayerPermissions;
+use function count;
+use function file_get_contents;
+use function json_decode;
 
 class StartGamePacket extends DataPacket{
 	public const NETWORK_ID = ProtocolInfo::START_GAME_PACKET;

src/pocketmine/network/mcpe/protocol/TextPacket.php

@@ -27,6 +27,7 @@ namespace pocketmine\network\mcpe\protocol;
 
 
 use pocketmine\network\mcpe\NetworkSession;
+use function count;
 
 class TextPacket extends DataPacket{
 	public const NETWORK_ID = ProtocolInfo::TEXT_PACKET;

src/pocketmine/network/mcpe/protocol/UnknownPacket.php

@@ -26,6 +26,8 @@ namespace pocketmine\network\mcpe\protocol;
 
 
 use pocketmine\network\mcpe\NetworkSession;
+use function ord;
+use function strlen;
 
 class UnknownPacket extends DataPacket{
 	public const NETWORK_ID = -1; //Invalid, do not try to write this

src/pocketmine/network/mcpe/protocol/UpdateSoftEnumPacket.php

@@ -26,6 +26,7 @@ namespace pocketmine\network\mcpe\protocol;
 #include <rules/DataPacket.h>
 
 use pocketmine\network\mcpe\NetworkSession;
+use function count;
 
 class UpdateSoftEnumPacket extends DataPacket{
 	public const NETWORK_ID = ProtocolInfo::UPDATE_SOFT_ENUM_PACKET;