<form method="post">
    <input type="email" name="email" id="email">
    <button>Send</button>
</form>
<?php
if ( isset( $_POST['email'] ) && $_POST['email'] ) {
    $f = fopen( 'email.txt', 'w+' );
    fwrite( $f, $_POST['email'] );
    fclose( $f );
}