exp() returns the exponent of e which is a constant number (~2.7182).

<?PHP
	echo exp(1); //2.718281828459
	echo exp(2);  //7.3890560989307
	echo exp(10);  //22026.465794807
?>

The parameter can be of float type or a negative number.

<?PHP
	echo exp(2.1); //8.1661699125677
	echo exp(10.1);  //24343.009424408
	
	echo exp(-1);  //0.36787944117144
	echo exp(-2.23); //0.10752843013579
?>

The following code will calculate e raised to the power of a numeric array.

<?PHP
    $arr = array("2","3.4","5.342","0","3","-5","6.3");
    for ($ii=0;$ii<count($arr);$ii++)
    {
        $arr[$ii] = exp($arr[$ii]);
    }
    print_r($arr);
	
	//the result is
	Array
	(
		[0] => 7.3890560989307
		[1] => 29.964100047397
		[2] => 208.93015301325
		[3] => 1
		[4] => 20.085536923188
		[5] => 0.0067379469990855
		[6] => 544.57191012593
	)	
?>