This file is the recommended way to add custom helper functions on perfex CRM. Here I added some functions to manibulate data on invoice module based on client's requirements.

my_functions_helper.php

<?php
/* TAWFIK */
/* Custom helper functions */

// Check if invoice can be edited
function is_invoice_editable( $id ) {
	if(get_option('disable_invoice_edit') == 1) {
		return check_invoice_is_draft( $id ) == true;
	}

	return true;
}

// Check if invoice status is draft
function check_invoice_is_draft( $id ): bool {
	$CI = &get_instance();

	$invoice = $CI->db->where( "id", $id )->get( db_prefix()  . 'invoices' )->row_array();
	if ( $invoice['status'] == 6 ) {
		return true;
	} else {
		return false;
	}
}

// Get invoice item tax rate
function get_item_tax_rate( $id ) {
	$CI = &get_instance();

	$tax = $CI->db->where( "itemid", $id )->get( db_prefix()  . 'item_tax' )->row_array();

	return $tax ? (float) $tax['taxrate'] : 0;
}

// Calculate item tax value
function calculate_item_tax( $item ) {
	// Tax rate
	$tax_rate = get_item_tax_rate( $item['id'] );

	// Item amount
	$amount = $item['qty'] * $item['rate'];

	// Calculate tax
	return ( $tax_rate * $amount ) / 100;
}

// Check if invoice number can be edited
function is_invoice_number_disabled() {
	if(get_option('disable_invoice_number_edit')) {
		return true;
	}

	return false;
}

// Check if invoice date can be edited
function is_invoice_date_disabled() {
	if(get_option('disable_invoice_date_edit')) {
		return true;
	}

	return false;
}

// Custom function for rate field decimals
function get_rate_decimals_place() {
	return 5;
}

// Filter function to add tax value beside tax percentage
hooks()->add_filter( 'item_tax_table_row', 'modify_invoice_item_tax', 10, 2 );
function modify_invoice_item_tax( $item_tax, $item ) {
	$tax_value = calculate_item_tax( $item );

	// Number format
	$tax_value = number_format($tax_value, 2);

	return "$item_tax ($tax_value)";
}

// Filter function to change invoice description column width
hooks()->add_filter( 'item_description_td_width', 'modify_item_description_td_width', 10, 1 );
function modify_item_description_td_width( $width ) {
	return 25;
}

// Filter function to update decimal places to 5 instead of 2
hooks()->add_filter( 'app_decimal_places', 'modify_app_decimal_places', 10, 1 );
function modify_app_decimal_places( $decimals ) {
	return 2;
}

// Filter function to format item rate
hooks()->add_filter( 'item_preview_rate', 'modify_item_preview_rate', 10, 1 );
function modify_item_preview_rate( $rate ) {
	return floatval($rate);
}