Find vowels | String

Exercise 4. Input string and count number of vowels. 1
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Find vowels | PHP</title>
</head>
<body>
    <?php
        $count=0;
        if(isset($_GET['submit'])){
            $s=$_GET['str'];
            for($i=0;$i<strlen($s);$i++) {
                if($s[$i]=='a'||$s[$i]=='e'||$s[$i]=='i'||$s[$i]=='o'||$s[$i]=='o'){
                    $count++;
                }
            }
        }
    ?>
    <h2>Input String and find vowels</h2>
    <form action="">
        <p><label for="">Enter String: </label> 
           <input type="text" name="str" value="<?php echo $_GET['str'];?>"></p>
        <p><button name='submit'>Find Vowels</button></p>
        <p><?php echo "Total Vowels = $count"; ?></p>
	</form>
</body>
</html>