Custom Validations

How to add custons validations

example: this email are exists

i’ve create a function check if email exists, but i need add custom validation “this email are exists”

check atual function

PHP

// Query
$sql = "SELECT FIELDS,STATUS FROM wp_evf_entries WHERE STATUS = 'publish' ";
$result = $conn->query($sql);

// request _POST
$email = '';
$email = $_POST['email'];

if ($result->num_rows > 0) {
	// output data of each row
	while ($row = $result->fetch_assoc()) {
		
		// create array only email field
		$varAux = explode(',', $row['FIELDS'], -1);
		$varAux2 = explode('"', $varAux[6], -1);

		// verify exist email
		if($email == $varAux2[3]){
			echo true;
			return false;
		}else{
			echo false;
		}
	}
}

JQUERY

<script>
		jQuery(document).ready(function($) {
			$("#evf-250-field_email").keyup(function() {
				email = jQuery("#evf-250-field_email").val();
				$.ajax({
					type      : 'post', 
					url       : 'validate_form.php', 
					data      :  {email:email}, 
					dataType  : 'html', 
					success: function(result){
						 if(result){
							 jQuery("#evf-250-field_email").after("<label class='evf-error'>Este email já consta no banco de dados, verifique sua caixa de entrada e spam</label>");
                    jQuery(".evf-submit").prop('disabled', true);
						 }
						else{
							jQuery("#evf-250-field_email").nextAll().remove();
                    jQuery(".evf-submit").prop("disabled", false);
						}
					}

				}); 
			});
		});
	</script>