abs() returns the absolute value of a number. It converts a negative number to positive. The parameter can be an integer or float.

<?PHP
	echo abs(4); //4
	echo abs(-4); //4
	echo abs(-3.3); //3.3
	echo abs(0); //0
?>

abs() cannot take an array as its parameter. You may need some typing to convert a numeric array.

<?PHP
	$arr = array("2","3.4","-5.342","0","3","54","-6.3");
	for ($ii=0;$ii<count($arr);$ii++)
	{
		$arr[$ii] = abs($arr[$ii]);
	}
	print_r($arr);  

	//the result is:
	Array
	(
		[0] => 2
		[1] => 3.4
		[2] => 5.342
		[3] => 0
		[4] => 3
		[5] => 54
		[6] => 6.3
	)	
?>

gmp_abs() has a similar function. It returns the absolute value of a number, or a string that can be converted to number, or a GMP (GNU Multiple Precision) number (large number). In order to use this function, you need first uncomment the ";php_gmp.dll" line in the file "php.ini".
   ...
   ;extension=php_bz2.dll
   ;extension=php_curl.dll
   ;extension=php_fileinfo.dll
   extension=php_gd2.dll
   ;extension=php_gettext.dll
   extension=php_gmp.dll
   ...

<?PHP
    echo gmp_abs("-761098327532"); //761098327532
?>