Reading attributes

A Read service call returns one or more attributes of one or more nodes. In OPC UA, a node's value is just one of its attributes — the Value attribute, ID 13. Reading the DisplayName, DataType, or BrowseName of the same node uses the same Read call with a different attribute ID.

Single attribute

use PhpOpcua\Client\Types\AttributeId;
use PhpOpcua\Client\Types\StatusCode;

$dv = $client->read('ns=2;s=Devices/PLC/Speed');

if (StatusCode::isGood($dv->statusCode)) {
    $speed = $dv->getValue();           // unwrapped Variant value
    $when  = $dv->sourceTimestamp;      // DateTimeImmutable on the device
}

The returned DataValue carries:

• value (Variant) — the typed value
• statusCode (int) — 0 for Good, non-zero for bad/uncertain
• sourceTimestamp (?DateTimeImmutable) — when the device sampled it
• serverTimestamp (?DateTimeImmutable) — when the server packaged the reply

getValue() unwraps the Variant to its native PHP value: bool, int, float, string, array (for Variant array values), or a decoded ExtensionObject body.

Multiple attributes — readMulti

readMulti() issues a single ReadRequest with N items. One round-trip, N results, in the same order:

use PhpOpcua\Client\Types\AttributeId;

$results = $client->readMulti([
    ['nodeId' => 'i=2261'],
    ['nodeId' => 'ns=2;s=Devices/PLC/Speed'],
    ['nodeId' => 'ns=2;s=Devices/PLC/Speed', 'attributeId' => AttributeId::DataType],
]);

[$productName, $speed, $speedType] = $results;

Each entry is a DataValue. The order matches the request order; gaps or reordering would be a protocol violation.

Fluent builder

readMulti() returns a ReadMultiBuilder when called with no arguments. The builder produces the same array structure with clearer intent at the call site:

$results = $client->readMulti()
    ->node('i=2261')->value()
    ->node('ns=2;s=Devices/PLC/Speed')->value()
    ->node('ns=2;s=Devices/PLC/Speed')->dataType()
    ->execute();

Builder methods:

The builder is a one-shot — call execute() exactly once.

Auto-batching

Servers advertise a MaxNodesPerRead operational limit. If you pass more items than the server allows, readMulti() automatically slices the request, issues N parallel reads, and stitches the results back in order. The server limit is discovered on connect() and overridable on the builder via setBatchSize(). See Connection · Endpoints and discovery.

Server BuildInfo helpers

The OPC UA spec mandates a small set of well-known nodes under Server.ServerStatus.BuildInfo. Reading them with read() works, but the library ships convenience methods:

echo $client->getServerProductName();        // ?string
echo $client->getServerManufacturerName();   // ?string
echo $client->getServerSoftwareVersion();    // ?string
echo $client->getServerBuildNumber();        // ?string
print_r($client->getServerBuildDate());      // ?DateTimeImmutable

$info = $client->getServerBuildInfo();       // BuildInfo DTO, single readMulti()

getServerBuildInfo() is the most efficient form — it batches all five attributes into a single readMulti call. Use it when you need them all.

Metadata cache

Reading the DataType or DisplayName of a node is common during discovery and rarely changes for the lifetime of a session. The library ships an opt-in metadata cache that stores non-Value attributes keyed by NodeId + attribute ID:

$client = ClientBuilder::create()
    ->setReadMetadataCache(true)
    ->connect('opc.tcp://plc.local:4840');

$client->read('ns=2;s=Tag', AttributeId::DataType);   // server hit
$client->read('ns=2;s=Tag', AttributeId::DataType);   // cache hit
$client->read('ns=2;s=Tag', AttributeId::DataType, refresh: true); // server hit

• Value (attribute 13) is never cached, even when the metadata cache is on.
• The cache uses the same PSR-16 backend as browse caching; see Observability · Caching for the storage codec and invalidation API.

Status codes vs exceptions

The exception path is for "the call could not complete"; the status-code path is for "the call completed, this specific node had a problem". Both are real failure modes — handle both.

See Reference · Exceptions for the full hierarchy.