- bin2hex: documentation ( source)
<?php
$key = openssl_pkey_get_public('-----BEGIN PUBLIC KEY-----
MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAt+S0ZxgyQ7BPcmz/JEa7
yEhcKDZTE9TgOF/9cW1w+quFvC43daYmyRpT3asYOm7YPGCmaQ7hUx9XKUUlEdXz
Zr1uvvDyFZdtS45+4nQ5DOI20mZoUHGV82rAMmvf5vote/JJu8Gt01ZUARfsMl+K
DtwpVDHN6LGPBOW8l8abktk1tL/oiwLSVrO2cM/IgBZETDkQpUaZxZx3yUcueEQ+
BFrtS3IYaEny938daQzElNdCaip0f68Ig0gOTPzwkzDOgyOhyjFRxx4aisGzIlXu
TFkqzIz7oC3JysgS5EhlwmsEIAelbZWpgc17HK2aWIzqlT99hB+kKv2fauxH/fgT
nQIDAQAB
-----END PUBLIC KEY-----');
$details = openssl_pkey_get_details($key);
echo bin2hex($details['rsa']['e']); // "010001"