Не сохраняется «0»

своих мозгов мало, поэтому изначальный код для вставки дополнительных полей «нагуглил»:

class trueMetaBox {
	function __construct($options) {
		$this->options = $options;
		$this->prefix = $this->options['id'] .'_';
		add_action( 'add_meta_boxes', array( &$this, 'create' ) );
		add_action( 'save_post', array( &$this, 'save' ), 1, 2 );
	}
	function create() {
		foreach ($this->options['post'] as $post_type) {
			if (current_user_can( $this->options['cap'])) {
				add_meta_box($this->options['id'], $this->options['name'], array(&$this, 'fill'), $post_type, $this->options['pos'], $this->options['pri']);
			}
		}
	}
	function fill(){
		global $post; $p_i_d = $post->ID;
		wp_nonce_field( $this->options['id'], $this->options['id'].'_wpnonce', false, true );
		?>
		<table class="form-table"><tbody><?php
		foreach ( $this->options['args'] as $param ) {
			if (current_user_can( $param['cap'])) {
			?><tr><?php
				if(!$value = get_post_meta($post->ID, $this->prefix .$param['id'] , true)) $value = $param['std'];
				switch ( $param['type'] ) {
					case 'text':{ ?>
						<th scope="row"><label for="<?php echo $this->prefix .$param['id'] ?>"><?php echo $param['title'] ?></label></th>
						<td>
							<input name="<?php echo $this->prefix .$param['id'] ?>" type="<?php echo $param['type'] ?>" id="<?php echo $this->prefix .$param['id'] ?>" value="<?php echo $value ?>" placeholder="<?php echo $param['placeholder'] ?>" class="regular-text" /><br />
							<span class="description"><?php echo $param['desc'] ?></span>
						</td>
						<?php
						break;							
					}
					case 'number':{ ?>
						<th scope="row"><label for="<?php echo $this->prefix .$param['id'] ?>"><?php echo $param['title'] ?></label></th>
						<td>
							<input name="<?php echo $this->prefix .$param['id'] ?>" type="<?php echo $param['type'] ?>" id="<?php echo $this->prefix .$param['id'] ?>" value="<?php echo $value ?>" placeholder="<?php echo $param['placeholder'] ?>" class="regular-text" /><br />
							<span class="description"><?php echo $param['desc'] ?></span>
						</td>
						<?php
						break;							
					}					
					case 'textarea':{ ?>
						<th scope="row"><label for="<?php echo $this->prefix .$param['id'] ?>"><?php echo $param['title'] ?></label></th>
						<td>
							<textarea name="<?php echo $this->prefix .$param['id'] ?>" type="<?php echo $param['type'] ?>" id="<?php echo $this->prefix .$param['id'] ?>" value="<?php echo $value ?>" placeholder="<?php echo $param['placeholder'] ?>" class="large-text" /><?php echo $value ?></textarea><br />
							<span class="description"><?php echo $param['desc'] ?></span>
						</td>
						<?php
						break;							
					}
					case 'checkbox':{ ?>
						<th scope="row"><label for="<?php echo $this->prefix .$param['id'] ?>"><?php echo $param['title'] ?></label></th>
						<td>
							<label for="<?php echo $this->prefix .$param['id'] ?>"><input name="<?php echo $this->prefix .$param['id'] ?>" type="<?php echo $param['type'] ?>" id="<?php echo $this->prefix .$param['id'] ?>"<?php echo ($value=='on') ? ' checked="checked"' : '' ?> />
							<?php echo $param['desc'] ?></label>
						</td>
						<?php
						break;							
					}
					case 'select':{ ?>
						<th scope="row"><label for="<?php echo $this->prefix .$param['id'] ?>"><?php echo $param['title'] ?></label></th>
						<td>
							<label for="<?php echo $this->prefix .$param['id'] ?>">
							<select name="<?php echo $this->prefix .$param['id'] ?>" id="<?php echo $this->prefix .$param['id'] ?>"><option>...</option><?php
								foreach($param['args'] as $val=>$name){
									?><option value="<?php echo $val ?>"<?php echo ( $value == $val ) ? ' selected="selected"' : '' ?>><?php echo $name ?></option><?php
								}
							?></select></label><br />
							<span class="description"><?php echo $param['desc'] ?></span>
						</td>
						<?php
						break;							
					}
				} 
			?></tr><?php
			}
		}
		?></tbody></table><?php
	}
	function save($post_id, $post){
		if ( !wp_verify_nonce( $_POST[ $this->options['id'].'_wpnonce' ], $this->options['id'] ) ) return;
		if ( !current_user_can( 'edit_post', $post_id ) ) return;
		if ( !in_array($post->post_type, $this->options['post'])) return;
		foreach ( $this->options['args'] as $param ) {
			if ( current_user_can( $param['cap'] ) ) {
				if ( isset( $_POST[ $this->prefix . $param['id'] ] ) && trim( $_POST[ $this->prefix . $param['id'] ] ) ) {
					update_post_meta( $post_id, $this->prefix . $param['id'], trim($_POST[ $this->prefix . $param['id'] ]) );
				} else {
					delete_post_meta( $post_id, $this->prefix . $param['id'] );
				}
			}
		}
	}
}

пробовал, почему-то не получается((