<?php

namespace Your_Namespace;

use Your_Namespace\Helpers as h;

final class My_Class {
	public static function __activation () {
		// This STATIC method is called automatically on plugin activation.
	}

	public static function __deactivation () {
		// This STATIC method is called automatically on plugin deactivation (not uninstall/deletation).
	}
}

// /classes/Helpers.php
class Helpers extends Core\Helpers {
    // put your custom helpers here
}

// /core/Helpers.php
class Helpers {
    ...
}

// /classes/Helpers.php
class Helpers {
    use Trait_Core_Helpers;
	use Trait_String_Helpers;
	use Trait_Template_Helpers;
	...
}

$data = [
	'name' => 'Luiz Bills',
	'nickname' => '',
	'age' => 55,
	'subscriber' => 'no'
];

$validation = h::validate(
	$data, 
	[
		'name' => [ 'required', 'string', 'min: 3', 'max: 100' ],
		'nickname' => [ 'optional', 'string', 'min: 1', 'max: 25' ],
		'age' => [ 'required', 'int', 'min: 18', 'max: 150' ],
		'subscriber' => [ 'required', 'string', 'in_list: yes,no' ],
	]
);

if ( ! $validation->success ) {
	$errors = $validation->get_errors();
	// do something
}

/**
 * The "required" validation rule
 * 
 * @param string  $value
 * @param array   $rule_values
 * @return string|null The message error
 */
function validation_rule_required ( $value, $rule_values = [] ) {
	if ( $value !== null && $value !== "" ) {
		return '{field} is required.';
	}
}

/**
 * The "in_list" validation rule
 */
function validation_rule_in_list ( $value, $rule_values = [] ) {
	if ( ! in_array( $value, $rule_values ) ) {
		return sprintf( '{field} must be one of these: %s', implode( ', ', $rule_values ) );
	}
}

/**
 * The "min" validation rule
 */
function validation_rule_min ( $value, $rule_values = [] ) {
	$min = (int) $rules_values[0];
	if ( is_string( $value ) && mb_strlen( $value ) <  $min ) {
		return sprintf( '{field} must be more than or equal to %d characters', $min );
	} elseif ( is_numeric( $value ) && $value < $min ) {
		return sprintf( '{field} must be greater than or equal to %d', $min );
	}
}

/**
 * The "optional" validation rule
 */
function validation_rule_optional ( $value, $rule_values = [] ) {
	if ( $value === null || $value === "" ) {
		return '##skip_validation##'; // special flag to skip other validations for this field
	}
}

h::add_validation_rule( 
	'char', // rule name
	function ( $value, $rule_values = [] ) { // rule callback
		if ( ! is_string( $value) || mb_strlen( $value ) !== 1 ) {
			return '{field} must be only 1 character';
		}
	}
);

// with shortcut
$deps[] = [
    'check' => 'function:WC',
    'message' => ...
];

// without shortcut (it will continue to be possible to pass a function to check)
$deps[] = [
    'check' => function () {
        return function_exists( 'WC' );
    }, 
    'message' => ...
];

// example
try {
    $min_expected_version = "7.4";
    $current_version = \preg_replace( '/[^0-9\.]/', '', PHP_VERSION );
    if ( \version_compare( $current_version, $min_expected_version, '>=' ) ) {
        throw new \Error( 'an useful message to administrators' );
    };
} catche ( \Throwable $e ) {
    // display a admin notice (only for administrators) and return
}

<?php
// loader.php

return [
    ...
    My_Class::get_instance(),
];