Replicate PHP's native date() formatting in JavaScript for many common format types

dateFormat.js

/**
 * Return a formated string from a date Object mimicking PHP's date() functionality
 *
 * format  string  "Y-m-d H:i:s" or similar PHP-style date format string
 * date    mixed   Date Object, Datestring, or milliseconds 
 *
 */
function dateFormat(format,date){
	
	if(!date || date === "")
	{ 
		date = new Date();
	}
	else if(typeof('date') !== 'object')
	{
		date = new Date(date.replace(/-/g,"/")); // attempt to convert string to date object	
	}
	
	var string = '',
		mo = date.getMonth(),   // month (0-11)
		m1 = mo+1,			    // month (1-12)
		dow = date.getDay(),    // day of week (0-6)
		d = date.getDate(),     // day of the month (1-31)
		y = date.getFullYear(), // 1999 or 2003
		h = date.getHours(),    // hour (0-23)
		mi = date.getMinutes(), // minute (0-59)
		s = date.getSeconds();  // seconds (0-59)
	
	for (var i = 0, len = format.length; i < len; i++) {
		switch(format[i])
		{
			case 'j': // Day of the month without leading zeros  (1 to 31)
				string+= d;
				break;
			
			case 'd': // Day of the month, 2 digits with leading zeros (01 to 31)
				string+= (d < 10) ? "0"+d : d;
				break;
			
			case 'l': // (lowercase 'L') A full textual representation of the day of the week
				var days = Array("Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday");
				string+= days[dow];
				break;
				
			case 'w': // Numeric representation of the day of the week (0=Sunday,1=Monday,...6=Saturday)
				string+= dow;
				break;
				
			case 'D': // A textual representation of a day, three letters
				days = Array("Sun","Mon","Tue","Wed","Thr","Fri","Sat");
				string+= days[dow];
				break;	
			
			case 'm': // Numeric representation of a month, with leading zeros (01 to 12)
				string+= (m1 < 10) ? "0"+m1 : m1;
				break;	
		
			case 'n': // Numeric representation of a month, without leading zeros (1 to 12)
				string+= m1;
				break;
			
			case 'F': // A full textual representation of a month, such as January or March 
				var months = Array("January","February","March","April","May","June","July","August","September","October","November","December");
				string+= months[mo];
				break;
				
			case 'M': // A short textual representation of a month, three letters (Jan - Dec)
				months = Array("Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec");
				string+= months[mo];
				break;
					
			case 'Y': // A full numeric representation of a year, 4 digits (1999 OR 2003)	
				string+= y;
				break;
				
			case 'y': // A two digit representation of a year (99 OR 03)
				string+= y.toString().slice(-2);
				break;
				
			case 'H': // 24-hour format of an hour with leading zeros (00 to 23)
				string+= (h < 10) ? "0"+h : h;
				break;
			
			case 'g': // 12-hour format of an hour without leading zeros (1 to 12)
				var hour = (h===0) ? 12 : h;
				string+= (hour > 12) ? hour -12 : hour;
				break;
				
			case 'h': // 12-hour format of an hour with leading zeros (01 to 12)
				hour = (h===0) ? 12 : h;
				hour = ( hour > 12) ? hour -12 : hour;
				string+= (hour < 10) ? "0"+hour : hour;
				break;
			
			case 'a': // Lowercase Ante meridiem and Post meridiem (am or pm)
				string+= (h < 12) ? "am" : "pm";
				break;		
			
			case 'i': // Minutes with leading zeros (00 to 59)
				string+= (mi < 10) ? "0"+mi : mi;
				break;
			
			case 's': // Seconds, with leading zeros (00 to 59)
				string+= (s < 10) ? "0"+s : s;
				break;
				
			case 'c': // ISO 8601 date (eg: 2012-11-20T18:05:54.944Z)
				string+= date.toISOString();
				break;		
				
			default:
				string+= format[i];
		}
	}

	return string;
}

The code by @kubiqsk handles these extra codes (missing from this implementation): "A, B, G, I, L, N, O, P, S, T, U, W, Z, e, o, r, t, v, z", however it doesn't handle escaped characters.

The follow code is an updated version of the above with suggested corrections above as well as handling escaped characters, if you don't care about escaped characters then I'd probably go with the kubiqsk one as it handles more codes.

/* Return a formatted string from a date Object mimicking PHP's date() functionality
 *
 * format  string  "Y-m-d H:i:s" or similar PHP-style date format string
 * date    mixed   Date Object, Datestring, or milliseconds
 *
 * NOTE
 * ~~~~~~~~~
 *     PHP: $today =       date('\i\t \i\s \t\h\e jS \d\a\y.');             // it is the 10th day.
 *      JS: today  = dateFormat('\\i\\t \\i\\s \\t\\h\\e jS \\d\\a\\y.');   // it is the 10 day.
 */
function dateFormat(format, date)
{
    if (!date || date === "")
        date = new Date();
    else if (typeof(date) !== 'object')
    {
        // attempt to convert string/number to date object
        try
        {
            if (typeof date === 'number')
               date = new Date(date);                    //timestamp
            else
               date = new Date(date.replace(/-/g, "/")); //probably string
        }
        catch (e)
        {
            date = new Date();                           //Can't handle whatever it was...
        }
    }

    var string = '',
        mo = date.getMonth(),   // month (0-11)
        m1 = mo+1,			    // month (1-12)
        dow = date.getDay(),    // day of week (0-6)
        d = date.getDate(),     // day of the month (1-31)
        y = date.getFullYear(), // 1999 or 2003
        h = date.getHours(),    // hour (0-23)
        mi = date.getMinutes(), // minute (0-59)
        s = date.getSeconds();  // seconds (0-59)

    for (var i = 0, len = format.length; i < len; i++) {
        switch(format[i])
        {
            case '\\': //Handle escaped characters
                i++;
                // console.log(format[i]);
                string+= format[i];
                break;
            case 'j': // Day of the month without leading zeros  (1 to 31)
                string+= d;
                break;

            case 'd': // Day of the month, 2 digits with leading zeros (01 to 31)
                string+= (d < 10) ? "0"+d : d;
                break;

            case 'l': // (lowercase 'L') A full textual representation of the day of the week
                var days = Array("Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday");
                string+= days[dow];
                break;

            case 'w': // Numeric representation of the day of the week (0=Sunday,1=Monday,...6=Saturday)
                string+= dow;
                break;

            case 'D': // A textual representation of a day, three letters
                days = Array("Sun","Mon","Tue","Wed","Thr","Fri","Sat");
                string+= days[dow];
                break;

            case 'm': // Numeric representation of a month, with leading zeros (01 to 12)
                string+= (m1 < 10) ? "0"+m1 : m1;
                break;

            case 'n': // Numeric representation of a month, without leading zeros (1 to 12)
                string+= m1;
                break;

            case 'F': // A full textual representation of a month, such as January or March
                var months = Array("January","February","March","April","May","June","July","August","September","October","November","December");
                string+= months[mo];
                break;

            case 'M': // A short textual representation of a month, three letters (Jan - Dec)
                months = Array("Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec");
                string+= months[mo];
                break;

            case 'Y': // A full numeric representation of a year, 4 digits (1999 OR 2003)
                string+= y;
                break;

            case 'y': // A two digit representation of a year (99 OR 03)
                string+= y.toString().slice(-2);
                break;

            case 'H': // 24-hour format of an hour with leading zeros (00 to 23)
                string+= (h < 10) ? "0"+h : h;
                break;

            case 'g': // 12-hour format of an hour without leading zeros (1 to 12)
                var hour = (h===0) ? 12 : h;
                string+= (hour > 12) ? hour -12 : hour;
                break;

            case 'h': // 12-hour format of an hour with leading zeros (01 to 12)
                hour = (h===0) ? 12 : h;
                hour = ( hour > 12) ? hour -12 : hour;
                string+= (hour < 10) ? "0"+hour : hour;
                break;

            case 'a': // Lowercase Ante meridiem and Post meridiem (am or pm)
                string+= (h < 12) ? "am" : "pm";
                break;

            case 'i': // Minutes with leading zeros (00 to 59)
                string+= (mi < 10) ? "0"+mi : mi;
                break;

            case 's': // Seconds, with leading zeros (00 to 59)
                string+= (s < 10) ? "0"+s : s;
                break;

            case 'c': // ISO 8601 date (eg: 2012-11-20T18:05:54.944Z)
                string+= date.toISOString();
                break;

            default:
                string+= format[i];
        }
    }

    return string;
}