(WIP) List of the only PHP functions that really need to exist

functions.php

<?php

array();
array_add();
array_change_key_case();
array_chunk();
array_column();
array_combine();
array_compact();
array_count();
array_count_values();
array_current();
array_diff();
array_diff_assoc();
array_diff_key();
array_end();
array_extract();
array_fill();
array_fill_keys();
array_filter();
array_flip();
array_has();
array_intersect();
array_intersect_assoc();
array_intersect_key();
array_intersect_uassoc();
array_intersect_ukey();
array_join();
array_key_exists();
array_map();
array_merge();
array_merge_recursive();
array_multisort();
array_pad();
array_pop();
array_previous();
array_rand();
array_reduce();
array_replace();
array_replace_recursive();
array_search();
array_shift();
array_shuffle();
array_slice();
array_sort();
array_sort_reverse();
array_splice();
array_split();
array_sum();
array_udiff();
array_udiff_assoc();
array_udiff_uassoc();
array_uintersect();
array_unique();
array_unset();
array_unshift();
array_values();
array_walk();
array_walk_recursive();

date();
date_add();
date_create_from_format();
date_create_immutable();
date_create_immutable_from_format();
date_default_timezone_get();
date_default_timezone_set();
date_diff();
date_format();
date_get_last_errors();
date_interval_format();
date_modify();
date_parse();
date_sub();
date_sunrise();
date_sunset();
date_time_set();
date_timestamp_get();
date_timestamp_set();
date_timezone_get();
date_timezone_set();

microtime();

time();
time_from_string();

timezone_abbreviations_list();
timezone_identifiers_list();
timezone_location_get();
timezone_name_from_abbr();
timezone_name_get();
timezone_offset_get();
timezone_open();
timezone_transitions_get();
timezone_version_get();

class_alias();
class_exists();
class_has_method();
class_has_property();
class_has_trait();

curl_close();
curl_copy_handle();
curl_error();
curl_error_number();
curl_escape();
curl_exec();
curl_file_create();
curl_get_info();
curl_init();
curl_multi_add_handle();
curl_multi_close();
curl_multi_error();
curl_multi_error_number();
curl_multi_exec();
curl_multi_get_content();
curl_multi_get_info();
curl_multi_init();
curl_multi_option();
curl_multi_remove_bundle();
curl_multi_select();
curl_option();
curl_pause();
curl_reset();
curl_share_close();
curl_share_error();
curl_share_error_number();
curl_share_init();
curl_share_option();
curl_unescape();
curl_version();

directory();
directory_change();
directory_change_root();
directory_close();
directory_get();
directory_open();
directory_read();
directory_remove();
directory_rewind();
directory_scan();

binary_open();
binary_read();
binary_write();
binary_delete();

file_change_group();
file_change_ownership();
file_copy();
file_create_symlink();
file_create_tempname();
file_delete();
file_get_contents();
file_get_directory();
file_get_group();
file_get_inode();
file_get_inode_modified();
file_get_last_access();
file_get_modified();
file_get_owner();
file_get_permissions();
file_get_size();
file_get_stats();
file_get_type();
file_lock();
file_move();
file_put_contents();
file_put_csv();
file_read_symlink();
file_touch();
file_unlock();

chgrp();
chmod();
chown();

glob();

get_called_class();
get_class();
get_class_methods();
get_class_vars();
get_date();
get_declared_classes();
get_declared_functions();
get_declared_interfaces();
get_declared_traits();
get_defined_vars();
get_function_arg();
get_function_args();
get_object_vars();
get_parent_class();
get_resource_type();
get_time_of_day();

is_array();
is_bool();
is_callable();
is_countable();
is_directory();
is_empty();
is_executable();
is_file();
is_float();
is_int();
is_iterable();
is_null();
is_numeric();
is_object();
is_readable();
is_resource();
is_scalar();
is_set();
is_string();
is_subclass_of();
is_symlink();
is_uploaded_file();
is_writeable();
serialize();
unserialize();
var_dump();
var_export();

filter_has_var();
filter_id();
filter_input();
filter_list();
filter_var();

function_call();
function_create();
function_exists();
function_static_call();

print_readable();
print_safe();

register_shutdown_function();
register_tick_function();
unregister_shutdown_function();
unregister_tick_function();

Note: Many function names have been rewritten for consistency. Please, feel free to add other functions that really need to exist. Classes (such as PDO and mysqli) have been left out because they're not pure functions.