PHP Manual: Add attribute values to current attributes

bool ldap_mod_add(
    LDAP\Connection $ldap,
    string $dn,
    array $entry,
    arraynull $controls = null
)

Adds one or more attribute values to the specified dn. To add a whole new object see ldap_add function.

ldap: An LDAP\Connection instance, returned by ldap_connect.
dn: The distinguished name of an LDAP entity.
entry: An associative array listing the attirbute values to add. If an attribute was not existing yet it will be added. If an attribute is existing you can only add values to it if it supports multiple values.
controls: Array of LDAP Controls to send with the request.

Returns true on success or false on failure.

Version	Description
8.1.0	The `ldap` parameter expects an LDAP\Connection instance now; previously, a valid `ldap link` resource was expected.
8.0.0	`controls` is nullable now; previously, it defaulted to `[]`.
7.3.0	Support for `controls` added

Note: This function is binary-safe.

ldap_mod_add