Write a debug log to the WP uploads directory. Use tail -f in a terminal to watch for changes on the fly

debug-log.php

<?php

/**
 * Write a debug log.
 *
 * @param mixed $msg A message to log.
 */
function sitecare_debug_log( $msg = null ) {
	if ( ! is_string( $msg ) ) {
		$msg = print_r( $msg, true );
	}

	$upload_dir = wp_upload_dir();
	$log_dir = "{$upload_dir['basedir']}/logs/";
	$log_file = "{$log_dir}sitecare-debug-log.txt";

	if ( ! is_dir( $log_dir ) ) {
		wp_mkdir_p( $log_dir );
	}

	$open_file = fopen( $log_file, 'a+' );

	if ( false !== $open_file ) {
		if ( null === $msg ) {
			fwrite( $open_file, date( "\r\nY-m-d H:I:s:\r\n" ) );
		} else {
			fwrite( $open_file, date( 'Y-m-d H:i:s - ' ) . $msg . "\r\n" );
		}
	}

	fclose( $open_file );
}