The client sends a x-elastic-client-meta header by default. This header is used to collect meta data about the versions of the components used by the client. For instance, a value of x-elastic-client-meta can be es=7.14.0-s,php=7.4.11,t=7.14.0-s,a=0,cu=7.68.0, where each value is the version of es=Elasticsearch, t is the transport version (same of client), a is asyncronouts (0=false by default) and cu=cURL.

If you would like to disable it you can use the setElasticMetaHeader() method, as follows:

$client = Elasticsearch\ClientBuilder::create()
    ->setElasticMetaHeader(false)
    ->build();

This is a special setting for the client that enables the port in the Host header. This setting has been introduced to prevent issues with HTTP proxy layers (see issue #993).

By default the port number is not included in the Host header. If you want you can enable it using the includePortInHostHeader() function, as follows:

$client = Elasticsearch\ClientBuilder::create()
    ->includePortInHostHeader(true)
    ->build();

Starting from version 7.13, Elasticsearch supports a compatibility header in Content-Type and Accept. The PHP client can be configured to emit the following HTTP headers:

Content-Type: application/vnd.elasticsearch+json; compatible-with=7
Accept: application/vnd.elasticsearch+json; compatible-with=7

which signals to Elasticsearch that the client is requesting 7.x version of request and response bodies. This allows upgrading from 7.x to 8.x version of Elasticsearch without upgrading everything at once. Elasticsearch should be upgraded first after the compatibility header is configured and clients should be upgraded second.

To enable this compatibility header, you need to create an ELASTIC_CLIENT_APIVERSIONING environment variable and set it to true or 1, before the Client class initialization.

In PHP you can set this environment variable as follows:

putenv("ELASTIC_CLIENT_APIVERSIONING=true");