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<?php
/**
 * Plugin Name: Преобразуент контент записи в plain text
 */

add_action( 'admin_menu', function () {
	add_management_page(
		'Форматирование записи',
		'Форматирование записи',
		'manage_options',
		'plain-text',
		'make_plain_text'
	);
} );

function make_plain_text() {
	$post_id = filter_input( INPUT_GET, 'post_id', FILTER_VALIDATE_INT );
	$post    = get_post( $post_id );
	?>
    <div class="wrap">
        <h2><?php echo get_admin_page_title() ?></h2>

        <form>
            <input type="text" name="post_id" value="<?php echo $post_id; ?>">
            <input type="hidden" name="page" value="plain-text">
            <button>Отправить</button>
        </form>

		<?php if ( $post ):
			$content = $post->post_content;
			$content = wpautop( $content );
			$content = str_replace( [ "\n", "\r", "\t" ], '', $content );
			?>
            <h3><?php echo esc_html( $post->post_title ); ?></h3>
            <textarea style="width: 100%; height: 600px;"><?php echo $content; ?></textarea>
		<?php else: ?>
            <p>Указанной записи нет!</p>
		<?php endif; ?>

    </div>
	<?php
}