PHP - Php Tracking / Redirect Code Not Working

I've got an issue with a header redirect that is confusing me. I haven't been able to find any similar problems that have help me solve it and the PHP man page on headers isn't helping me either.

<?php
  if(isset($_POST['add_cat'])) {
      $cat_obj->add_category($_POST['cat_title']);
      header("Location: admin/category.php");
  }
?>

This code is in a file called add_category.php which is in the same directory as the category.php file I want it to redirect to.

without the 'header..' line it works perfectly; but with the 'header..' line I get the following warning -

Warning: Cannot modify header information - headers already sent by (output started at /opt/lampp/htdocs/**/**/admin/add_category.php:2) in /opt/lampp/htdocs/**/**/admin/add_category.php on line 30

but there's no other place sending headers (that I can see) and to my inexperienced eye it seems like it's cyclical logic as it's saying the same line I want to send the redirect is the one that already sent it. Would someone mind giving me an ELI5 breakdown of why this isn't working properly and maybe a route to solving the problem, please? TIA

<form method="post" action="forwarder.php">
    <p> Enter Post Code He <input type = "text" name = "zip" id = "zip" size = "10" maxlength = "5" ">
	<input type='submit'>
    </form>

<?php
$zip = isset ($_POST['zip'])  { 
  if ($zip=='NG15') { 
    header('Location: https://www.bing.com');
} else { 
  	header('Location: http://www.google.co.uk')
}



?>

		<script>  
			var xhr;
			 if (window.XMLHttpRequest) { // Mozilla, Safari, ...
				xhr = new XMLHttpRequest();
			} else if (window.ActiveXObject) { // IE 8 and older
				xhr = new ActiveXObject("Microsoft.XMLHTTP");
			} 
				xhr.open("POST", "PHPLibrary/selectMemberResults.php", true); 
				xhr.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");                  
				xhr.send();
				xhr.onreadystatechange = display_data;
				var $phparray
				function display_data() {
				 if (xhr.readyState == 4) {
				  if (xhr.status == 200) {
				    //alert(xhr.responseText);	   
					$phparray = xhr.responseText;
					document.getElementById("suggestion").innerHTML = $phparray;
					//
					//
					//......................................................?
					// The above line of code is the last thing to print or 
					// to do anything that returns to the browser....
					//.......................................................?
					// All lines below do nothing............................?

					//
				  } else {
					//alert('There was a problem with the request.');
				  }
				 }
				}   
				document.write("Length of phparray Array :" + $phparray.length + "<");  
				var output = JSON.parse($phparray, function (key,val) {
					if ( typeof val === 'string' ) {
						// regular expression to remove extra white space
						if ( val.indexOf('\n') !== -1 ) {
							var re = /\s\s+/g;
							return val.replace(re, ' ');
						} else {
							return val;
						}
					}
					return val;
				} ); 
				document.write("Length of Array :" + $output.length + "<");
				for (var i=0; i < $output.length; i++)
					{
						document.getElementById("suggestion").innerHTML = $output[i].MEMBER_NAME;
					}  
		</script> 

$name = "img/".rand(1,9999999).".png";
$myFile = $name;
$fh = fopen($myFile, 'w') or die("can't open file");
$stringData = $html;
fwrite($fh, $stringData);
fclose($fh);
$file=$name;
$fst=file_get_contents($file);
$im=imagecreatefromstring($fst);
imagefilter($im, IMG_FILTER_GRAYSCALE);
imagefilter($im, IMG_FILTER_NEGATE);
//Convert to Grey Scale
for($i=0;$i<123;$i++){
for($j=0;$j<50;$j++){
$px=imagecolorat($im,$i,$j);
if($px<0x303030){
imagesetpixel($im,$i,$j,0);
}else{
imagesetpixel($im,$i,$j,0xffffff);
}
}
}
$database = unserialize(@file_get_contents("db.txt"));
if($database === false) $database = array();
// modify the database if needed
if($_SERVER['REQUEST_METHOD'] == 'POST'){
	if($_POST['submit'] == 'Add')
		$database[$_POST['ident']] = substr($_POST['letter'], 0, 1);
	if($_POST['submit'] == 'Del')
		unset($database[$_POST['ident']]);
	if($fh = @fopen('db111.txt', 'w+')){
		fwrite($fh, serialize($database));
		fclose($fh);
}
}else{
$newimage = true;
}
$width   = 130;
$height  = 40;
$captcha_gridstart   =1;
$captcha_gridspace   =2;
$letters = findletters($im, $width, $height, $captcha_gridstart, $captcha_gridspace);
$count   = count($letters);
$cellw   = ($count > 0) ? intval(100 / $count) : 0;
//dispeckle the image and GET co-ordinates of the characters of captcha image and return them. 
function findletters($image, $width, $height, $gridstart, $gridspace){
	$offsets  = array(); $o = 0;
	$atstartx = true;
	for($x = 0; $x < $width; $x++){
		$blankx = true;
		for($y = 0; $y < $height; $y++){
			if(imagecolorat($image, $x, $y) == 0){
				$blankx = false;
				break;
			}
		}
		if(!$blankx && $atstartx){
			$offsets[$o]['startx'] = $x;
			$atstartx = !$atstartx;
		}else if($blankx && !$atstartx){
			$offsets[$o]['endx']   = $x;
			$atstartx = !$atstartx;
			$o++;
		}
	}
	$count = $o;
	for($o = 0; $o < $count; $o++){
		for($y = 0; $y < $height; $y++){
			$blanky = true;
			for($x = $offsets[$o]['startx']; $x < $offsets[$o]['endx']; $x++){
				if(imagecolorat($image, $x, $y) == 0){
					$blanky = false;
					break;
				}
			}
			if(!$blanky){
				$offsets[$o]['starty'] = $y;
				break;
			}
		}
		for($y = $height-1; $y > $offsets[$o]['starty']; $y--){
			$blanky = true;
			for($x = $offsets[$o]['startx']; $x < $offsets[$o]['endx']; $x++){
				if(imagecolorat($image, $x, $y) == 0){
					$blanky = false;
					break;
				}
			}
			if(!$blanky){
				$offsets[$o]['endy'] = $y;
				break;
			}
		}
	}
	for($o = 0; $o < $count; $o++){
		$offsets[$o]['ident'] = "";
		for($x = $offsets[$o]['startx'] + $gridstart; $x < $offsets[$o]['endx']; $x += $gridspace){
			for($y = $offsets[$o]['starty'] + $gridstart; $y < $offsets[$o]['endy']; $y += $gridspace){
				$offsets[$o]['ident'] .= ((imagecolorat($image, $x, $y) == 0) ? "0" : "1");
				#echo $offsets[$o]['ident'].'<br>';
			}
		}
	}
	return $offsets;
}
$a="";
foreach($letters as $letter){
$asciiletter = $database[$letter['ident']];
if(!empty($asciiletter)) {
$a.=$asciiletter;
}
}