paytm_with_gravity_form

add_filter( 'gform_validation', 'ask_custom_validation' );

function ask_custom_validation( $validation_result ) {

	global $email, $wpdb;
	$table_lead			 = $wpdb->prefix . 'rg_lead';
	$table_lead_detail	 = $wpdb->prefix . 'rg_lead_detail';
	// new gravity form
	$rg					 = new RGFormsModel();

	// grab the form
	$form	 = $validation_result[ "form" ];
	$form_id = $form[ 'id' ];
	// only validate the vault form 
	// Please replace form title with your form title
	if ( $form[ 'title' ] == "Registration with Paytm Payment" ) {

		foreach ( $form[ 'fields' ] as &$field ) {
			// store email field in variable
			// Please replace form field id with your form field id 
			if ( $field[ 'id' ] == "2" && $field[ 'type' ] == 'email' ) {
				$email = rgpost( "input_{$field[ 'id' ]}" );
			}
			$field_id = $field[ 'id' ];
			// check the uid field
			if ( $field[ 'label' ] == "Email" ) {

				// search gravity forms for this email
				$sql	 = "SELECT * FROM $table_lead as lead
  JOIN $table_lead_detail as lead_detail ON lead.id = lead_detail.lead_id
  WHERE lead.form_id=$form_id
  AND field_number = $field_id
  AND value LIKE '%$email%' AND payment_status = 'Success'";
				$result	 = $wpdb->get_results( $sql, ARRAY_A );
				// if email already exists
				if ( count( $result ) > 0 ) {
					// set the form validation to false
					$validation_result[ "is_valid" ]				 = false;
					// specify the first field to be invalid and provide custom validation message
					$form[ "fields" ][ 1 ][ "failed_validation" ]	 = true;
					$form[ "fields" ][ 1 ][ "validation_message" ]	 = __( 'This email is already registered!', 'text_domain' );

					// update the form in the validation result with the form object you modified
					$validation_result[ "form" ] = $form;
				}
			}
		}
	}

	return $validation_result;
}