php四则运算：中缀表达式转后缀表达式例子

<?php 
	/** 
	 * 将输入的字符按照中缀表达式转后缀表达式的规则处理 
	 * @param $str 输入的字符 
	 * @param $stack 栈 
	 * @param $newStrList 新的表达式 
	 */ 
	function suffix($str, &$stack, &$newStrList){ 
	    //如果是数字则输出 
	    if(is_numeric($str)){ 
	        $newStrList .= $str . &#39; &#39;; 
	    } 
	    //如果是左括号则入栈 
	    else if($str == &#39;(&#39;){ 
	        $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 == &#39;)&#39;){ 
	        while($arrPop = array_pop($stack)){ 
	            if($arrPop == &#39;(&#39;){ 
	                break; 
	            } 
	            $newStrList .= $arrPop . &#39; &#39;; 
	        }  
	    } 
	    //如果是加减乘除则判断与栈顶符号优先级 
	    else if(in_array($str, array(&#39;+&#39;, &#39;-&#39;, &#39;*&#39;, &#39;/&#39;)) && count($stack) > 0){ 
	        $key  = (count($stack) - 1); 
	        if(in_array($stack[$key], array(&#39;+&#39;, &#39;-&#39;, &#39;*&#39;, &#39;/&#39;))){ 
	            //该符号优先级不高于栈顶符号的 
	            if(checkPriority($str, $stack[$key]) != 1){ 
	                for($i=$key; $i>=0; $i--){ 
	                    if($stack[$i] == &#39;(&#39;){ 
	                        break; 
	                    } 
	                    $newStrList .= $stack[$i] . &#39; &#39;; 
	                    unset($stack[$i]); 
	                    $stack = array_values($stack); 
	                } 
	            } 
	        } 
	        //本次的符号入栈 
	        $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 &#39;+&#39;: 
	        case &#39;-&#39;: 
	            if($operatorB == &#39;+&#39; || $operatorB == &#39;-&#39;){ 
	                return 0; 
	            }else if($operatorB == &#39;*&#39; || $operatorB == &#39;/&#39;){ 
	                return -1; 
	            } 
	            break; 
	        case &#39;*&#39;: 
	        case &#39;/&#39;: 
	            if($operatorB == &#39;+&#39; || $operatorB == &#39;-&#39;){ 
	                return 1; 
	            }else if($operatorB == &#39;*&#39; || $operatorB == &#39;/&#39;){ 
	                return 0; 
	            } 
	            break; 
	        default: 
	            exit(&#39;error&#39;); 
	    } 
	} 
	//栈 
	$stack = array(); 
	//待转换的表达式 
	$strList = &#39;9 + ( 3 - 1 ) * 3 + 10 / 2&#39;; 
	//新的表达式 
	$newStrList = &#39;&#39;; 
	$strList = explode(&#39; &#39;, $strList); 
	foreach($strList as $str){ 
	    if($str != &#39; &#39;){ 
	        suffix($str, $stack, $newStrList); 
	    } 
	} 
	//数组反转 
	while($s = array_pop($stack)){ 
	    $newStrList .= $s . &#39; &#39;; 
	} 
	echo $newStrList;