<?php
$input = '5+6*2+1=';
$inputLenght = mb_strlen($input);

$number = 0;
$result = 0;
$op = ' ';

for($i = 0; $i < $inputLenght; $i++){
	$char = mb_substr($input, $i, 1);
	if($char == '*' || $char == '+' || $char == '-' || $char == '=') {

    switch($op){
    	case '+':
    		$result = intval("$result") + intval("$number");
    	case '*':
    		$result = intval("$result") * intval("$number");
    	case '-':
    		$result = intval("$result") - intval("$number");
    	case '=':
    		$result = intval("$result") + intval("$number");
    	
    		
    }
    echo "op = [$op] , result = [$result], number = [$number], char = [$char] \n";
    $number = 0;
	$op = $char;
	
	
	}elseif (is_numeric($char)) {
		
	    $number = ($number * 10) + $char; 
	    
		
	}else {
	    echo " WRONG SYMBOL: {$char} \n";	
	}
}


    echo " Ответ: $input $result";