A simple PHP BBCode Parser function

php_bbcode_parser.php

<?php

/**
* A simple PHP BBCode Parser function
*
* @author afsalrahim
* @contributors dzhaugasharov, luckydevilru, kylehulton
**/

//BBCode Parser function

function showBBcodes($text) {
    // IMPORTANT: Sanitize user input to prevent security vulnerabilities.
    $charset = 'UTF-8'; // Specify your character set
    $text = htmlspecialchars($text, ENT_QUOTES, $charset);

    // BBcode array
    $find = array(
        '~\[b\](.*?)\[/b\]~s',
        '~\[i\](.*?)\[/i\]~s',
        '~\[u\](.*?)\[/u\]~s',
        '~\[quote\](.*?)\[/quote\]~s',
        '~\[size=(.*?)\](.*?)\[/size\]~s',
        '~\[color=(.*?)\](.*?)\[/color\]~s',
        '~\[url\]((?:ftp|https?)://.*?)\[/url\]~s',
        '~\[url=([^"><]*?)\](.*?)\[/url\]~s', // Added url with label
        '~\[img\](https?://.*?\.(?:jpg|jpeg|gif|png|bmp))\[/img\]~s',
        '~\[h([1-6])\](.*?)\[/h\1\]~s', // Added H# parsing
    );

    // HTML tags to replace BBcode
    $replace = array(
        '<b>$1</b>',
        '<i>$1</i>',
        '<span style="text-decoration:underline;">$1</span>',
        '<pre>$1</'.'pre>',
        '<span style="font-size:$1px;">$2</span>',
        '<span style="color:$1;">$2</span>',
        '<a href="$1">$1</a>',
        '<a href="$1">$2</a>', // Added url with label
        '<img src="$1" alt="" />',
        '<h$1>$2</h$1>' // Added H# parsing
    );

    // Replacing the BBcodes with corresponding HTML tags
    return preg_replace($find, $replace, $text);
}

// How to use the above function:

$bbtext = "This is [b]bold[/b] and this is [u]underlined[/u] and this is in [i]italics[/i] with a [color=red] red color[/color] and a [h1]Heading 1[/h1] and a link [url=https://ya.ru]test[/url]";
$htmltext = showBBcodes($bbtext);
echo $htmltext;

?>

Alert XSS

I've improved a little

public static function replaceBBcodes($text)
    {
    	$text = strip_tags($text);
		// BBcode array
		$find = array(
			'~\[b\](.*?)\[/b\]~s',
			'~\[i\](.*?)\[/i\]~s',
			'~\[u\](.*?)\[/u\]~s',
			'~\[quote\]([^"><]*?)\[/quote\]~s',
			'~\[size=([^"><]*?)\](.*?)\[/size\]~s',
			'~\[color=([^"><]*?)\](.*?)\[/color\]~s',
			'~\[url\]((?:ftp|https?)://[^"><]*?)\[/url\]~s',
			'~\[img\](https?://[^"><]*?\.(?:jpg|jpeg|gif|png|bmp))\[/img\]~s'
		);
		// HTML tags to replace BBcode
		$replace = array(
			'<b>$1</b>',
			'<i>$1</i>',
			'<span style="text-decoration:underline;">$1</span>',
			'<pre>$1</'.'pre>',
			'<span style="font-size:$1px;">$2</span>',
			'<span style="color:$1;">$2</span>',
			'<a href="$1">$1</a>',
			'<img src="$1" alt="" />'
		);
		// Replacing the BBcodes with corresponding HTML tags
		return preg_replace($find, $replace, $text);
	}

if url like this [url=https://ya.ru]test[/url]

'~\[url=([^"><]*?)\](.*?)\[/url\]~s',
and
'<a href="$1">$2</a>',

to Parse [H#] headers

add
'~\[h([^"><]*?)\](.*?)\[/h([^"><]*?)\]~s',
and
'<h$1>$2</h$3>',
in the appropriate order 👍🏻

This is a tidier alternative on a much later thought :D
'~\[h([1-6])\](.*?)\[/h\1\]~s',

'<h$1>$2</h$1>',