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]);
 			}
 			
 			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;
 	}
 }