WordPress at a glance
function is not described

validate_plugin() WP 2.5.0

Validate the plugin path.

Checks that the main plugin file exists and is a valid plugin. See validate_file().

No Hooks.

Return

WP_Error/Int. 0 on success, WP_Error on failure.

Usage

validate_plugin( $plugin );
$plugin(string) (required)
Path to the main plugin file from plugins directory.

Code of validate plugin: wp-admin/includes/plugin.php VER 5.0.3

<?php
function validate_plugin($plugin) {
	if ( validate_file($plugin) )
		return new WP_Error('plugin_invalid', __('Invalid plugin path.'));
	if ( ! file_exists(WP_PLUGIN_DIR . '/' . $plugin) )
		return new WP_Error('plugin_not_found', __('Plugin file does not exist.'));

	$installed_plugins = get_plugins();
	if ( ! isset($installed_plugins[$plugin]) )
		return new WP_Error('no_plugin_header', __('The plugin does not have a valid header.'));
	return 0;
}