<?php
 
error_reporting(-1);
mb_internal_encoding('utf-8');

$input = '7594+2366/5453.543+65*657.467-155=';

$regexp = '/[+]|[-]|[*]|\\/|[=]/';
$numbers = preg_split ($regexp, $input, 0, PREG_SPLIT_NO_EMPTY);

foreach($numbers as $key => $number) {
	$number = floatval($number);
}

$regexp = '/[0-9]+[.]*[0-9]+/';
$input = preg_replace($regexp, '', $input);

$regexp = '//';
$actions = preg_split ($regexp, $input, 0, PREG_SPLIT_NO_EMPTY);

$sum = $numbers[0];
$amountOfNumbers = count($numbers);
for ($i = 0; $i < $amountOfNumbers-1; $i++) {
	
	if ($actions[$i] == '+') {
		$sum = $sum + $numbers[$i+1];
	} elseif ($actions[$i] == '-') {
		$sum = $sum - $numbers[$i+1];
	} elseif ($actions[$i] == '*') {
		$sum = $sum * $numbers[$i+1];
	} elseif ($actions[$i] == '/') {
		$sum = $sum / $numbers[$i+1];
	}
}

echo "Ответ: {$sum}";