Take a look at this..

I use this in an accounting system where you can write math expressions in amount input fields..

Examples

$Cal = new Field_calculate();

$result = $Cal->calculate('5+7'); // 12
$result = $Cal->calculate('(5+9)*5'); // 70
$result = $Cal->calculate('(10.2+0.5*(2-0.4))*2+(2.1*4)'); // 30.4

Code

class Field_calculate {
    const PATTERN = '/(?:\-?\d+(?:\.?\d+)?[\+\-\*\/])+\-?\d+(?:\.?\d+)?/';

    const PARENTHESIS_DEPTH = 10;

    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){
        $compute = create_function('', 'return '.$input.';');

        return 0 + $compute();
    }

    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;
    }
}

I recently created a PHP package that provides a math_eval helper function. It does exactly what you need, without the need to use the potentially unsafe eval function.

You just pass in the string version of the mathematical expression and it returns the result.

$two   = math_eval('1 + 1');
$three = math_eval('5 - 2');
$ten   = math_eval('2 * 5');
$four  = math_eval('8 / 2');

You can also pass in variables, which will be substituted if needed.

$ten     = math_eval('a + b', ['a' => 7, 'b' => 3]);
$fifteen = math_eval('x * y', ['x' => 3, 'y' => 5]);

Link: https://github.com/langleyfoxall/math_eval

Using eval function is very dangerous when you can't control the string argument.

Try Matex for safe Mathematical formulas calculation.

Finding a sweetspot between the dangers of eval and the limitless calculation possibilities I suggest checking the input for only numbers, operators and brackets:

if (preg_match('/^[0-9\+\-\*\/\(\)\.]+$/', $mathString)) {
    $value = eval('return
    ' . $mathString . ';');
} else {
    throw new \Exception('Invalid calc() value: ' . $mathString);
}

It's still easy to use yet relatively save. And it can handle any basic math calulation like (10*(1+0,2)) which isn't possible with most of the mentioned solutions here.

Solved!

<?php 
function evalmath($equation)
{
    $result = 0;
    // sanitize imput
    $equation = preg_replace("/[^a-z0-9+\-.*\/()%]/","",$equation);
    // convert alphabet to $variabel 
    $equation = preg_replace("/([a-z])+/i", "\$$0", $equation); 
    // convert percentages to decimal
    $equation = preg_replace("/([+-])([0-9]{1})(%)/","*(1\$1.0\$2)",$equation);
    $equation = preg_replace("/([+-])([0-9]+)(%)/","*(1\$1.\$2)",$equation);
    $equation = preg_replace("/([0-9]{1})(%)/",".0\$1",$equation);
    $equation = preg_replace("/([0-9]+)(%)/",".\$1",$equation);
    if ( $equation != "" ){
        $result = @eval("return " . $equation . ";" );
    }
    if ($result == null) {
        throw new Exception("Unable to calculate equation");
    }
    echo $result;
   // return $equation;
}


$a = 2;
$b = 3;
$c = 5;
$f1 = "a*b+c";

$f1 = str_replace("a", $a, $f1);
$f1 = str_replace("b", $b, $f1);
$f1 = str_replace("c", $c, $f1);

evalmath($f1);
/*if ( $equation != "" ){

    $result = @eval("return " . $equation . ";" );
}
if ($result == null) {

    throw new Exception("Unable to calculate equation");
}
echo $result;*/
?>

This method has two major drawbacks:

• Security, php script is being evaluated by the eval function. This is bad, especially when the user wants to inject malicious code.

• Complexity

I created this, check it out: Formula Interpreter

How does it work ?

First, create an instance of FormulaInterpreter with the formula and its parameters

$formulaInterpreter = new FormulaInterpreter("x + y", ["x" => 10, "y" => 20]);

Use the execute() method to interpret the formula. It will return the result:

echo $formulaInterpreter->execute();

in a single line

echo (new FormulaInterpreter("x + y", ["x" => 10, "y" => 20]))->execute();

Examples

# Formula: speed = distance / time
$speed = (new FormulaInterpreter("distance/time", ["distance" => 338, "time" => 5]))->execute() ;
echo $speed;


#Venezuela night overtime (ordinary_work_day in hours): (normal_salary * days_in_a_work_month)/ordinary_work_day
$parameters = ["normal_salary" => 21000, "days_in_a_work_month" => 30, "ordinary_work_day" => 8];
$venezuelaLOTTTArt118NightOvertime = (new FormulaInterpreter("(normal_salary/days_in_a_work_month)/ordinary_work_day", $parameters))->execute();
echo $venezuelaLOTTTArt118NightOvertime;


#cicle area
$cicleArea = (new FormulaInterpreter("3.1416*(radio*radio)", ["radio" => 10]))->execute();
echo $cicleArea;

About the formulas

1. It must contain at least two operands and an operator.
2. Operands' name could be in upper or lower case.
3. By now, math functions as sin, cos, pow… are not included. I'm working to include them.
4. If your formula is not valid, you will get an error message like: Error, your formula (single_variable) is not valid.
5. Parameters' values must be numeric.

You can improve it if you want to!

eval Evaluates the given code as PHP. Meaning that it will execute the given paremeter as a PHP piece of code.

To correct your code, use this :

$ma ="print (2+10);";
eval($ma);

Using eval function

protected function getStringArthmeticOperation($value, $deduct)
{
    if($value > 0){
        $operator = '-';
    }else{
        $operator = '+';
    }
    $mathStr = '$value $operator $deduct';
    eval("\$mathStr = \"$mathStr\";");
    $userAvailableUl = eval('return '.$mathStr.';');
    return $userAvailableUl;
}

$this->getStringArthmeticOperation(3, 1); //2

I found this cool method on Github, hope it helps someone.

<?php 
function evaluate_math_string($str) {
    $__eval = function ($str) use(&$__eval){
        $error = false;
        $div_mul = false;
        $add_sub = false;
        $result = 0;

        $str = preg_replace('/[^\d.+\-*\/()]/i','',$str);
        $str = rtrim(trim($str, '/*+'),'-');

        /* lets first tackle parentheses */
        if ((strpos($str, '(') !== false &&  strpos($str, ')') !== false)) {
            $regex = '/\(([\d.+\-*\/]+)\)/';
            preg_match($regex, $str, $matches);
            if (isset($matches[1])) {
                return $__eval(preg_replace($regex, $__eval($matches[1]), $str, 1));
            }
        }

        /* Remove unwanted parentheses */
        $str = str_replace(array('(',')'), '', $str);
        /* now division and multiplication */
        if ((strpos($str, '/') !== false ||  strpos($str, '*') !== false)) {
            $div_mul = true;
            $operators = array('*','/');
            while(!$error && $operators) {
                $operator = array_pop($operators);
                while($operator && strpos($str, $operator) !== false) {
                   if ($error) {
                      break;
                   }
                   $regex = '/([\d.]+)\\'.$operator.'(\-?[\d.]+)/';
                   preg_match($regex, $str, $matches);
                   if (isset($matches[1]) && isset($matches[2])) {
                          if ($operator=='+') $result = (float)$matches[1] + (float)$matches[2];
                          if ($operator=='-') $result = (float)$matches[1] - (float)$matches[2]; 
                          if ($operator=='*') $result = (float)$matches[1] * (float)$matches[2]; 
                          if ($operator=='/') {
                             if ((float)$matches[2]) {
                                $result = (float)$matches[1] / (float)$matches[2];
                             } else {
                                $error = true;
                             }
                          }
                          $str = preg_replace($regex, $result, $str, 1);
                          $str = str_replace(array('++','--','-+','+-'), array('+','+','-','-'), $str);
                   } else {
                      $error = true;
                   }
                }
            }
        }
         
        if (!$error && (strpos($str, '+') !== false ||  strpos($str, '-') !== false)) {
            //tackle duble negation 
            $str = str_replace('--', '+', $str);
            $add_sub = true;
            preg_match_all('/([\d\.]+|[\+\-])/', $str, $matches);
            if (isset($matches[0])) {
                $result = 0;
                $operator = '+';
                $tokens = $matches[0];
                $count = count($tokens);
                for ($i=0; $i < $count; $i++) { 
                    if ($tokens[$i] == '+' || $tokens[$i] == '-') {
                        $operator = $tokens[$i];
                    } else {
                        $result = ($operator == '+') ? ($result + (float)$tokens[$i]) : ($result - (float)$tokens[$i]);
                    }
                }
            }
        }
        if (!$error && !$div_mul && !$add_sub) {
             $result = (float)$str;
        }
        return $error ? 0 : $result;
    };
    return $__eval($str);
}
?>

Here is an example how to use it:

<?php 
require('evaluate_math.php');

/* Test cases */
$tests = array(
    '-3',
    '+5', 
    '5+2', 
    '-5-1', 
    '5-1', 
    '2 + 2.5 + 3 / 3 + 3 * 3', 
    '5 + 10 /-2 - 5 *2',
    '((5 - 2.5) + 2) * ((5 - 2.5) + 2)',
    '(((5 - 2.5) + 2) * ( 10 / 5))',
    '5-(-2)'
);
foreach($tests as $test) {
    echo $test . '= '. evaluate_math_string($test) . '<br />';
}
?>

AND FINALLY HERE IS A REF LINK TO GITHUB https://github.com/samirkumardas/evaluate_math_string

An eval'd expression should end with ";"

Try this :

$ma ="2+10;";
$p = eval($ma);
print $p;

By the way, this is out of scope but the 'eval' function won't return the value of the expression. eval('2+10') won't return 12. If you want it to return 12, you should eval('return 2+10;');