From http://codex.wordpress.org/Editing_wp-config.php

wp-config.php

/**
 * This will log all errors notices and warnings to a file called debug.log in
 * wp-content only when WP_DEBUG is true. if Apache does not have write permission,
 * you may need to create the file first and set the appropriate permissions (i.e. use 666).
 */

define( 'WP_DEBUG', true ); // Or false
if ( WP_DEBUG ) {
    define( 'WP_DEBUG_LOG', true );
    define( 'WP_DEBUG_DISPLAY', false );
    @ini_set( 'display_errors', 0 );
}