Following is a list of some of the most useful PHP functions that a PHP developer will need at any point in his career. If you see any room for improvement, or if you can add something than go ahead and comment and I will definately give it a look for possible inclusion.

Strip Slashes

Removes slashes contained in a string or in an array

/**
 * Strip Slashes
 *
 * Removes slashes contained in a string or in an array
 *
 * @param	mixed	string or array
 * @return	mixed	string or array
 */	

function strip_slashes($str)
{
	if (is_array($str))
	{
		foreach ($str as $key => $val)
		{
			$str[$key] = strip_slashes($val);
		}
	}
	else
	{
		$str = stripslashes($str);
	}

	return $str;
}

Strip Quotes

Removes single and double quotes from a string

/**
 * Strip Quotes
 *
 * Removes single and double quotes from a string
 *
 * @param	string
 * @return	string
 */	

function strip_quotes($str)
{
	return str_replace(array('"', "'"), '', $str);
}

Quotes to Entities

Converts single and double quotes to entities

/**
 * Quotes to Entities
 *
 * Converts single and double quotes to entities
 *
 * @param	string
 * @return	string
 */	

function quotes_to_entities($str)
{
	return str_replace(array("\'","\"","'",'"'), array("'",""","'","""), $str);
}

Reduce Double Slashes

Converts double slashes in a string to a single slash, except those found in http:// .

/**
 * Reduce Double Slashes
 *
 * Converts double slashes in a string to a single slash,
 * except those found in http://
 *
 * http://www.mypaaji.com//index.php
 *
 * becomes:
 *
 * http://www.mypaaji.com/index.php
 *
 * @param	string
 * @return	string
 */	

function reduce_double_slashes($str)
{
	return preg_replace("#([^:])//+#", "\\1/", $str);
}

Reduce Multiples

Reduces multiple instances of a particular character. Example: Sunny, Yuvi,, Ashwini, Sneha becomes: Sunny, Yuvi, Ashwini, Sneha

/**
 * Reduce Multiples
 *
 * Reduces multiple instances of a particular character.  Example:
 *
 * Sunny, Yuvi,, Ashwini, Sneha
 *
 * becomes:
 *
 * Sunny, Yuvi, Ashwini, Sneha
 *
 * @param	string
 * @param	string	the character you wish to reduce
 * @param	bool	TRUE/FALSE - whether to trim the character from the beginning/end
 * @return	string
 */	

function reduce_multiples($str, $character = ',', $trim = FALSE)
{
	$str = preg_replace('#'.preg_quote($character, '#').'{2,}#', $character, $str);

	if ($trim === TRUE)
	{
		$str = trim($str, $character);
	}

	return $str;
}

Create a Random String

Useful for generating passwords or hashes.

/**
 * Create a Random String
 *
 * Useful for generating passwords or hashes.
 *
 * @param	string 	type of random string.  Options: alunum, numeric, nozero, unique
 * @param	integer	number of characters
 * @return	string
 */
function random_string($type = 'alnum', $len = 
{
	switch($type)
	{
		case 'alnum'	:
		case 'numeric'	:
		case 'nozero'	:

				switch ($type)
				{
					case 'alnum'	:	$pool = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
						break;
					case 'numeric'	:	$pool = '0123456789';
						break;
					case 'nozero'	:	$pool = '123456789';
						break;
				}

				$str = '';
				for ($i=0; $i < $len; $i++)
				{
					$str .= substr($pool, mt_rand(0, strlen($pool) -1), 1);
				}
				return $str;
		  break;
		case 'unique' : return md5(uniqid(mt_rand()));
		  break;
	}
}

Word Censoring Function

Supply a string and an array of disallowed words and any matched words will be converted to #### or to the replacement word you’ve submitted.

/**
 * Word Censoring Function
 *
 * Supply a string and an array of disallowed words and any
 * matched words will be converted to #### or to the replacement
 * word you've submitted.
 *
 * @param	string	the text string
 * @param	string	the array of censoered words
 * @param	string	the optional replacement value
 * @return	string
 */	

function word_censor($str, $censored, $replacement = '')
{
	if ( ! is_array($censored))
	{
		return $str;
	}

	$str = ' '.$str.' ';
	foreach ($censored as $badword)
	{
		if ($replacement != '')
		{
			$str = preg_replace("/\b(".str_replace('\*', '\w*?', preg_quote($badword)).")\b/i", $replacement, $str);
		}
		else
		{
			$str = preg_replace("/\b(".str_replace('\*', '\w*?', preg_quote($badword)).")\b/ie", "str_repeat('#', strlen('\\1'))", $str);
		}
	}

	return trim($str);
}

Word Limiter

Limits a string to X number of words.

/**
 * Word Limiter
 *
 * Limits a string to X number of words.
 *
 * @param	string
 * @param	integer
 * @param	string	the end character. Usually an ellipsis
 * @return	string
 */
function word_limiter($str, $limit = 100, $end_char = '…')
{
	if (trim($str) == '')
	{
		return $str;
	}

	preg_match('/^\s*+(?:\S++\s*+){1,'.(int) $limit.'}/', $str, $matches);

	if (strlen($str) == strlen($matches[0]))
	{
		$end_char = '';
	}

	return rtrim($matches[0]).$end_char;
}

Character Limiter

Limits the string based on the character count. Preserves complete words so the character count may not be exactly as specified.

/**
 * Character Limiter
 *
 * Limits the string based on the character count.  Preserves complete words
 * so the character count may not be exactly as specified.
 *
 * @param	string
 * @param	integer
 * @param	string	the end character. Usually an ellipsis
 * @return	string
 */
function character_limiter($str, $n = 500, $end_char = '…')
{
	if (strlen($str) < $n)
	{
		return $str;
	}

	$str = preg_replace("/\s+/", ' ', str_replace(array("\r\n", "\r", "\n"), ' ', $str));

	if (strlen($str) <= $n)
	{
		return $str;
	}

	$out = "";
	foreach (explode(' ', trim($str)) as $val)
	{
		$out .= $val.' ';
		if (strlen($out) >= $n)
		{
			return trim($out).$end_char;
		}
	}
}

That’s it guys I will keep updating this post with more handy functions, Please contribute if you have any.