Description:
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From manual page: http://www.php.net/function.openssl-encrypt
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The example code has a few items which if not downright security issues, at least is bad practice from a security POV.

issue 1: 
The IV should be included in the HMAC calculation, otherwise this is left open to manipulation. 

issue 2: 
When decrypting the MAC should be verified before attempting to decrypt the contents, otherwise any timing impacts on the decryption itself may still leak information about the correctness of the guess.

Because of issue 1 it would probably also be better to reorder the data in the example so it is $iv.$ciphertext_raw.$hmac instead.