Self-contained log function.

log.php

<?php
/**
 * Log messages, timestamps and  values throughout a page load and return them at the end for output at the bottom of
 * the page or for storage with a single database touch.
 *
 * @param string $message Log message.
 * @param mixed  $value   Log value.
 * @param string $format  Optional. Return type. Either text or array. Default array.
 *
 * @return array|string
 */
function log( $message, $value, $format = 'array' ) {
	static $log = [];

	$log[] = [
		'message' => $message,
		'value' => $value,
		'time' => current_time( 'mysql' ),
	];

	if ( 'text' === $format ) {
		$return = '';

		foreach ( $log as $line ) {
			$return .= printf( "[%s] %s: %s \n", esc_html( $line['time'] ), esc_html( $line['message'] ), esc_html( $line['value'] ) );
		}

		return $return;
	}

	return $log;
}