What PHP 8.4 changes for WordPress plugins

These are the functions, constants and settings that PHP 8.4 removes, deprecates or no longer ships, with what to use instead. A removed function or constant stops a plugin with a fatal error when that code runs. A deprecated one keeps working and logs a notice.

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No longer ships with PHP 8.4

imap_open() and 74 more. It works only if your host installs the imap extension from PECL. Ask your host before you upgrade.
imap_open(), imap_reopen(), imap_close(), imap_is_open(), imap_num_msg(), imap_num_recent(), imap_headers(), imap_headerinfo(), imap_rfc822_parse_headers(), imap_rfc822_write_address(), imap_rfc822_parse_adrlist(), imap_body(), imap_fetchtext(), imap_bodystruct(), imap_fetchbody(), imap_fetchmime(), imap_savebody(), imap_fetchheader(), imap_fetchstructure(), imap_gc(), imap_expunge(), imap_delete(), imap_undelete(), imap_check(), imap_listscan(), imap_scan(), imap_scanmailbox(), imap_mail_copy(), imap_mail_move(), imap_mail_compose(), imap_createmailbox(), imap_create(), imap_renamemailbox(), imap_rename(), imap_deletemailbox(), imap_subscribe(), imap_unsubscribe(), imap_append(), imap_ping(), imap_base64(), imap_qprint(), imap_8bit(), imap_binary(), imap_utf8(), imap_status(), imap_mailboxmsginfo(), imap_setflag_full(), imap_clearflag_full(), imap_sort(), imap_uid(), imap_msgno(), imap_list(), imap_listmailbox(), imap_lsub(), imap_listsubscribed(), imap_getsubscribed(), imap_getmailboxes(), imap_fetch_overview(), imap_alerts(), imap_errors(), imap_last_error(), imap_search(), imap_utf7_decode(), imap_utf7_encode(), imap_utf8_to_mutf7(), imap_mutf7_to_utf8(), imap_mime_header_decode(), imap_thread(), imap_timeout(), imap_get_quota(), imap_get_quotaroot(), imap_set_quota(), imap_setacl(), imap_getacl(), imap_mail()
pspell_new() and 18 more. It works only if your host installs the pspell extension from PECL. Ask your host before you upgrade.
pspell_new(), pspell_new_personal(), pspell_new_config(), pspell_check(), pspell_suggest(), pspell_store_replacement(), pspell_add_to_personal(), pspell_add_to_session(), pspell_clear_session(), pspell_save_wordlist(), pspell_config_create(), pspell_config_runtogether(), pspell_config_mode(), pspell_config_ignore(), pspell_config_personal(), pspell_config_dict_dir(), pspell_config_data_dir(), pspell_config_repl(), pspell_config_save_repl()
oci_define_by_name() and 123 more. It works only if your host installs the oci8 extension from PECL. Ask your host before you upgrade.
oci_define_by_name(), ocidefinebyname(), oci_bind_by_name(), ocibindbyname(), oci_bind_array_by_name(), oci_free_descriptor(), ocifreedesc(), oci_lob_save(), ocisavelob(), oci_lob_import(), ocisavelobfile(), oci_lob_load(), ociloadlob(), oci_lob_read(), oci_lob_eof(), oci_lob_tell(), oci_lob_rewind(), oci_lob_seek(), oci_lob_size(), oci_lob_write(), oci_lob_append(), oci_lob_truncate(), oci_lob_erase(), oci_lob_flush(), ocisetbufferinglob(), ocigetbufferinglob(), oci_lob_copy(), oci_lob_is_equal(), oci_lob_export(), ociwritelobtofile(), oci_new_descriptor(), ocinewdescriptor(), oci_rollback(), ocirollback(), oci_commit(), ocicommit(), oci_field_name(), ocicolumnname(), oci_field_size(), ocicolumnsize(), oci_field_scale(), ocicolumnscale(), oci_field_precision(), ocicolumnprecision(), oci_field_type(), ocicolumntype(), oci_field_type_raw(), ocicolumntyperaw(), oci_field_is_null(), ocicolumnisnull(), oci_execute(), ociexecute(), oci_cancel(), ocicancel(), oci_fetch(), ocifetch(), ocifetchinto(), oci_fetch_all(), ocifetchstatement(), oci_fetch_object(), oci_fetch_row(), oci_fetch_assoc(), oci_fetch_array(), oci_free_statement(), ocifreestatement(), oci_free_cursor(), ocifreecursor(), oci_close(), ocilogoff(), oci_new_connect(), ocinlogon(), oci_connect(), ocilogon(), oci_pconnect(), ociplogon(), oci_error(), ocierror(), oci_num_fields(), ocinumcols(), oci_parse(), ociparse(), oci_get_implicit_resultset(), oci_set_prefetch(), ocisetprefetch(), oci_set_prefetch_lob(), oci_set_client_identifier(), oci_set_edition(), oci_set_module_name(), oci_set_action(), oci_set_client_info(), oci_set_db_operation(), oci_set_call_timeout(), oci_password_change(), ocipasswordchange(), oci_new_cursor(), ocinewcursor(), oci_result(), ociresult(), oci_client_version(), oci_server_version(), ociserverversion(), oci_statement_type(), ocistatementtype(), oci_num_rows(), ocirowcount(), oci_free_collection(), ocifreecollection(), oci_collection_append(), ocicollappend(), oci_collection_element_get(), ocicollgetelem(), oci_collection_assign(), oci_collection_element_assign(), ocicollassignelem(), oci_collection_size(), ocicollsize(), oci_collection_max(), ocicollmax(), oci_collection_trim(), ocicolltrim(), oci_new_collection(), ocinewcollection(), oci_register_taf_callback(), oci_unregister_taf_callback()

Deprecated

mysqli_ping(), mysqli_kill(), mysqli_refresh(). Run the equivalent SQL statement.
lcg_value(). Use Random\Randomizer::getFloat().
xml_set_object(). Pass a callable that includes the object to the xml_set_ functions.
E_STRICT, CURLOPT_BINARYTRANSFER, SOAP_FUNCTIONS_ALL, SUNFUNCS_RET_TIMESTAMP, SUNFUNCS_RET_STRING, SUNFUNCS_RET_DOUBLE. The constant is deprecated.
session.sid_length, session.sid_bits_per_character, session.use_trans_sid, session.trans_sid_tags, session.trans_sid_hosts, session.referer_check. The setting is deprecated.

Bumpcheck also checks syntax changes, such as array offsets with curly braces and constructors named after their class. The full list is on the plugin page.