calcNumbers

$calc = array(
    '+' => 'bcadd',
    '-' => 'bcsub',
    '*' => 'bcmul'
);
$mathString = '5*19+45-6*7+21';
$numbers = preg_split('/[\+\-*]+/', $mathString);
$operators = preg_split('/[\d]+/', $mathString, -1, PREG_SPLIT_NO_EMPTY);
$numOperators =count($operators);
for ($i = 0; $i< $numOperators; $i++) {
    if ($operators[$i] == '*') {
        $numbers[$i] = call_user_func('bcmul', $numbers[$i], $numbers[$i+1]);
        $numbers[$i+1] = 0;
        $operators[$i] = '+';
    }
}
$sum = $numbers[0];
for ($i = 0; $i< $numOperators; $i++) {
    $sum = call_user_func($calc[$operators[$i]], $sum, $numbers[$i+1]);
}
echo "Result: ".$sum;