<?php
mb_internal_encoding('utf-8');
$input='243+6743-78*2=';
$inputLength=strlen($input);
$number=0;
$result=0;
$op='+';

function doMath($op,$result,$number) {

	return $op==='+'?$result+$number:($op==='-'?$result-$number:$result*$number);
}


for ($i=0;$i<$inputLength;$i++) {

    $char=mb_substr($input,$i,1);

    //validation and coercion step

    if (is_numeric($char)) {
        $char=intval($char);
    }
    elseif (!is_numeric($char)&&!in_array($char,['-','+','=','*'])) {
            echo "Enter correct value!"; exit();
        }

    //math

	if ($char=='+'||$char=='-'||$char=='*'||$char=='=') {

		$result=doMath($op,$result,$number);

       	$number=0;
		$op=$char;

	} else {

		$number=$number*10+$char;

	}

    //when equation sign reached

	if ($op=='=') {
		echo "". $result;
	}
}
?>