PHP string encryption

function djEncryptString($string, $key) 
{
	// $string is a random string to be encrypted.
	// $key is a randomized string that must contain all 
	// the characters found in $string as an absolute minimum.
	
	$key_chars = str_split($key);
	$string_chars = str_split($string);
	$key_coords = '';
	$key_coord_lengths = '';

	foreach($string_chars as $s_char) {
		for ($loop_a = 0; $loop_a < count($key_chars); $loop_a++) {
			if ($s_char == $key_chars[$loop_a]) {
				$key_coords .= (string)$loop_a;
				$key_coord_lengths .= (string)strlen($loop_a);
			}
		}
	}
	
	// Returns a string of coordinates of where the string 
	// characters are to be found within $key as well as the 
	// length of every coordinate since the string is a line 
	// of random numbers.
	
	return $key_coords .'-'. $key_coord_lengths;
}

function djDecryptString($enc_str, $key) 
{	
	// $enc_str is the output from djEncryptString().
	// $key is the same key as used to encrypt with.
	
	$key_chars = str_split($key);
	$enc_data = explode('-', $enc_str);
	$key_coords = $enc_data[0];
	$key_coord_lengths = str_split($enc_data[1]);
	
	$s = '';
	foreach ($key_coord_lengths as $c) {
		$s .= $key_chars[(int)substr($key_coords, 0, (int)$c)];
		$key_coords = substr($key_coords, (int)$c);
	}

	return $s;
}

// Create a random key to be used by both functions.
$key = str_shuffle('abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890. ');

$original_string = 'This is a random string.';
$encrypted_string = djEncryptString($original_string, $key);

echo 'Key used: '. $key .'<br><br>';
echo 'Original string: '. $original_string .'<br><br>';
echo 'Encrypted string: '. $encrypted_string .'<br><br>';
echo 'Decrypted string: '. djDecryptString($encrypted_string, $key);

Key used: L2RZ6oMfcCjpWIuPxKsT5rSzteq.Dmln0O17d8YUX FgV4yNBiEvbJH3Q9AakhwG

Original string: This is a random string.

Encrypted string: 19614918414918415941215931365294118242149314327-222222222222221222222222

Decrypted string: This is a random string.