四则运算表达式转换为后缀表达式

<?php
function suffix($str, &$stack, &$newStrList){

//如果是数字则输出
if(is_numeric($str)){
 $newStrList .= $str . ' ';
}
//如果是左括号则入栈
 else if($str == '('){
 $stack[] = $str;
}
//如果是右括号则将最近的左括号之前的所有数据出栈
 else if($str == ')'){
 while($arrPop = array_pop($stack)){
 if($arrPop == '('){
break;
}
 $newStrList .= $arrPop . ' ';
}
}
//如果是加减乘除则判断与栈顶符号优先级
 else if(in_array($str, array('+', '-', '*', '/')) && count($stack) > 0){
 $key = (count($stack) - 1);
 if(in_array($stack[$key], array('+', '-', '*', '/'))){
 if(checkPriority($str, $stack[$key]) != 1){
 for($i=$key; $i>=0; $i--){
 if($stack[$i] == '('){
break;
}
 $newStrList .= $stack[$i] . ' ';
unset($stack[$i]);
 $stack = array_values($stack);
}
 $stack[] = $str;
}else{
 $stack[] = $str;
}
}else{
 $stack[] = $str;
}
}else{
 $stack[] = $str;
}
}

/**
 * 判断运算符的优先级
 * @param $operatorA
 * @param $operatorB
 * @return A大于B返回1，A等于B返回0，A小于B返回-1
*/
function checkPriority($operatorA, $operatorB){
switch($operatorA){
 case '+':
 case '-':
 if($operatorB == '+' || $operatorB == '-'){
 return 0;
 }else if($operatorB == '*' || $operatorB == '/'){
 return -1;
}
break;
 case '*':
 case '/':
 if($operatorB == '+' || $operatorB == '-'){
 return 1;
 }else if($operatorB == '*' || $operatorB == '/'){
 return 0;
}
break;
default:
exit('error');
}
}

$stack = array();
$newStrList = '';
$strList = '9 + ( 3 - 1 ) * 3 + 10 / 2';
$strList = explode(' ', $strList);
foreach($strList as $str){
 if($str == ' '){
continue;
}
 suffix($str, $stack, $newStrList);
}
$stack = array_reverse($stack);
foreach($stack as $s){
 $newStrList .= $s . ' ';
}
echo $newStrList;