近期一个小项目需要用到公式运算， 所以就进行一些了解，以下内容均属于个人经验。

在PHP中实现公式表达式四则运算大概有两种方法:

1)使用系统函数eval

2)将表达式转换成逆波兰表达式进行计算。

//使用系统函数eval

$str = 'L*((k-J)-(C+k))/M';

$param = array('L' => 0.5, 'k' => 2, 'J' => 1, 'C' => 6, 'M' => 4);

$str2 = '';

for($i = 0; $i < strlen($str); $i++) {

$tmp = substr($str, $i, 1);

if (array_key_exists($tmp, $param)) {

$str2 .= $param[$tmp];

} else {

$str2 .= $tmp;

}

}

eval("\$str = $str2;");

printf('%5.2d', $str);

/*End of php*/

/**

* math_rpn

*

* 实现逆波兰式算法

*

* @author sparkHuang 260558820@qq.com

* @version RPN 1.0.0

*

*/

class math_rpn {

//初始的计算表达式

private $_expression = '';

//处理后的逆波兰表达式

private $_rpnexp = array();

//模拟栈结构的数组

private $_stack = array('#');

//正则判断

//private $_reg = '/^([A-Za-z0-9\(\)\+\-\*\/])*$/';

//优先级

private $_priority = array('#' => 0, '(' => 10, '+' => 20, '-' => 20, '*' => 30, '/' => 30);

//四则运算

private $_operator = array('(', '+', '-', '*', '/', ')');

public function __construct($expression) {

$this->_init($expression);

}

private function _init($expression) {

$this->_expression = $expression;

}

public function exp2rpn() {

$len = strlen($this->_expression);

for($i = 0; $i < $len; $i++) {

$char = substr($this->_expression, $i, 1);

if ($char == '(') {

$this->_stack[] = $char;

continue;

} else if ( ! in_array($char, $this->_operator)) {

$this->_rpnexp[] = $char;

continue;

} else if ($char == ')') {

for($j = count($this->_stack); $j >= 0; $j--) {

$tmp = array_pop($this->_stack);

if ($tmp == "(") {

break;

} else {

$this->_rpnexp[] = $tmp;

}

}

continue;

} else if ($this->_priority[$char] <= $this->_priority[end($this->_stack)]) {

$this->_rpnexp[] = array_pop($this->_stack);

$this->_stack[] = $char;

continue;

} else {

$this->_stack[] = $char;

continue;

}

}

for($i = count($this->_stack); $i >= 0; $i--) {

if (end($this->_stack) == '#') break;

$this->_rpnexp[] = array_pop($this->_stack);

}

return $this->_rpnexp;

}

}

$expression = "(A*(B+C)-E+F)*G";

var_dump($expression);

$mathrpn = new math_rpn($expression);

var_dump($mathrpn->exp2rpn());

/*End of php*/