The manual is not constistent at this point. On one hand, it describes the return as digest/fingerprint of a cert, on the other hand it says:
bool openssl_x509_fingerprint( … )
On PHP 5.6.5, I only get the bool return (1 or 0).
openssl_x509_fingerprint — Calculates the fingerprint, or digest, of a given X.509 certificate
$hash_algorithm= "sha1" [, bool
openssl_x509_fingerprint() returns the digest of
x509 as a string.
See Key/Certificate parameters for a list of valid values.
The hash algorithm to use, e.g. "md5" or "sha1"
When set to
TRUE, outputs raw binary data.
FALSE outputs lowercase hexits.
Returns a string containing the calculated certificate fingerprint as lowercase hexits unless
raw_output is set to
TRUE in which case the raw binary representation of the message digest is returned.
FALSE on failure.