Field_calculate.php

<?php

if (!defined('BASEPATH')) {
    exit('No direct script access allowed');
}

/**
 * This class describes a field calculate.
 * reff : https://stackoverflow.com/a/27077376/10351006
 * 
 * 
 * how to use class->calculate(input string)
 */
class Field_calculate
{
    const PATTERN = '/(?:\-?\d+(?:\.?\d+)?[\+\-\*\/])+\-?\d+(?:\.?\d+)?/';
    const PARENTHESIS_DEPTH = 10;

    public function __construct()
    {

    }

    public function calculate($input)
    {
        if (strpos($input, '+') != null || strpos($input, '-') != null || strpos($input, '/') != null || strpos($input, '*') != null)
        {
            //  Remove white spaces and invalid math chars
            $input = str_replace(',', '.', $input);
            $input = preg_replace('[^0-9\.\+\-\*\/\(\)]', '', $input);

            //  Calculate each of the parenthesis from the top
            $i = 0;
            while (strpos($input, '(') || strpos($input, ')'))
            {
                $input = preg_replace_callback('/\(([^\(\)]+)\)/', 'self::callback', $input);

                $i++;
                if ($i > self::PARENTHESIS_DEPTH)
                {
                    break;
                }
            }

            //  Calculate the result
            if (preg_match(self::PATTERN, $input, $match))
            {
                return $this->compute($match[0]);
            }
            // To handle the special case of expressions surrounded by global parenthesis like "(1+1)"
            if (is_numeric($input))
            {
                return $input;
            }

            return 0;
        }

        return $input;
    }

    private function compute($input) {
        // only for php v5.6
        // deprecated on php v7.2+
        // 
        // $compute = create_function('', 'return ' . $input . ';');
        // return 0 + $compute;

        // support php 5.6+

        return 0 + eval('return '.$input.';');
    }

    private function callback($input)
    {
        if (is_numeric($input[1]))
        {
            return $input[1];
        }
        elseif (preg_match(self::PATTERN, $input[1], $match))
        {
            return $this->compute($match[0]);
        }

        return 0;
    }
}