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<?php
/**
 * Gravity Wiz // Gravity Forms // Hide Next & Previous Buttons
 */
// Update "123" to your form ID.
add_filter( 'gform_next_button_123', 'gw_hide_button' );
add_filter( 'gform_previous_button_123', 'gw_hide_button' );
function gw_hide_button( $markup ) {
	return sprintf( '<div style="position:absolute;visibility:hidden;">%s</div>', $markup );
}

<?php
/**
 * Gravity Wiz // Gravity Forms // Hide Next & Previous Buttons on Specific Page
 * http://gravitywiz.com/
 */
// Update "123" to your form ID.
add_filter( 'gform_previous_button_123', 'gw_hide_button' );
add_filter( 'gform_next_button_123', 'gw_hide_button' );
function gw_hide_button( $markup ) {
	// Update "123" to your form ID. Update "4" to the page you wish to hide the button.
	if ( rgpost( 'gform_source_page_number_123' ) - 1 == 4 ) {
		return sprintf( '<div style="position:absolute;visibility:hidden;">%s</div>', $markup );
	}
	return $markup;
}