To complete the examples below that use proc_open to encrypt a string using GPG, here is a decrypt function:
<?php
function gpg_decrypt($string, $secret) {
 $homedir = ''; // path to you gpg keyrings
 $tmp_file = '/tmp/gpg_tmp.asc' ; // tmp file to write to 
 file_put_contents($tmp_file, $string);
 $text = '';
 $error = '';
 $descriptorspec = array(
 0 => array("pipe", "r"), // stdin
 1 => array("pipe", "w"), // stdout
 2 => array("pipe", "w") // stderr ?? instead of a file
 );
 $command = 'gpg --homedir ' . $homedir . ' --batch --no-verbose --passphrase-fd 0 -d ' . $tmp_file . ' ';
 $process = proc_open($command, $descriptorspec, $pipes);
 if (is_resource($process)) {
 fwrite($pipes[0], $secret);
 fclose($pipes[0]);
 while($s= fgets($pipes[1], 1024)) {
 // read from the pipe
 $text .= $s;
 }
 fclose($pipes[1]);
 // optional:
 while($s= fgets($pipes[2], 1024)) {
 $error .= $s . "\n";
 }
 fclose($pipes[2]);
 }
 
 file_put_contents($tmp_file, '');
 
 if (preg_match('/decryption failed/i', $error)) {
 return false;
 } else {
 return $text;
 }
}
?>