A useful error logging function intended for use with WordPress. Simply add to your wp-config.php, or other non-version controlled PHP file.

pj_error_log.function.php

<?php
if ( ! defined( 'PJID' ) ) define( 'PJID', uniqid() );
if ( ! defined( 'PJLOG' ) ) define( 'PJLOG', ABSPATH . 'pj.log' );
function pj_error_log(){

	// Get line numbers and such
	$caller = array_shift( debug_backtrace() );
	$prefix = implode( '::', [
		PJID,
		$caller['file'],
		$caller['line'],
		$caller['function'],
	] );
	$msgs = [];

	// First we check if we're being asked to print a pretty message
	if ( func_num_args() == 2 && is_string( func_get_arg(0) ) ) {
		$msgs[] = sprintf(
			'%s:%s',
			func_get_arg(0),
			var_export( func_get_arg(1), true )
		);
	}

	// Otherwise, just print each message/var accordingly.
	else if ( ! empty( func_get_args() ) ) {
		foreach ( func_get_args() as $arg ) {
			$msgs[] = ( is_string( $arg ) ) ? $arg : var_export( $arg, true );
		}
	}

	$log = fopen( PJLOG, 'a' );
	array_map( function($msg) use ($log, $prefix) {
		$msg = sprintf( '%s - %s', $prefix, $msg );
		fwrite( $log, $msg . PHP_EOL );
	}, $msgs );
	fclose( $log );
}