Here are some samples of its usage.

Versions comparison :

$ rpmdev-vercmp 2.0.14-22.el7_0 2.0.14.1-35.el7_6
2.0.14-22.el7_0 < 2.0.14.1-35.el7_6
php > var_dump(rpmvercmp('2.0.14-22.el7_0', '2.0.14.1-35.el7_6'));
int(-1)

Information about a package / file:

$ rpm -qp remi-release-32.rpm
remi-release-32-1.fc32.remi.noarch

php > print_r(rpminfo("remi-release-32.rpm"));
Array
(
    [Name] => remi-release
    [Version] => 32
    [Release] => 1.fc32.remi
    [Summary] => Configuration for remi repository
    [Arch] => noarch
)

With more details

$ rpm -qip remi-release-32.rpm
Name        : remi-release
Version     : 32
Release     : 1.fc32.remi
Architecture: noarch
...

php > print_r(rpminfo("remi-release-32.rpm", true)); 
Array
(
    [Headeri18ntable] => Array
        (
            [0] => C
            [1] => fr
        )
    [Sigsize] => 16816
    [Sigpgp] => 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
    [Sigmd5] => 86dc03cedbebcc9a8a334763d642685a
    [Rsaheader] => 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
    [Sha1header] => c528cb3308e69a4b9baa9a1e23a6541b06254f0a
    [Sha256header] => 9494000c44cf0665d12395631c21a90e4a44c4b7f37a8b3b1bbc798a0b99e1dd
    [Name] => remi-release
    [Version] => 32
    [Release] => 1.fc32.remi
...
    [Payloaddigestalgo] => 8
    [IsSource] =>
)

Lot of available fields (all those retrieved by the library API).

Information about an installed package:

$ rpm -q kernel-5.5.8
kernel-5.5.8-200.fc31.x86_64
php > print_r(rpmdbinfo("kernel-5.5.8"));
Array
(
    [0] => Array
        (
            [Name] => kernel
            [Version] => 5.5.8
            [Release] => 200.fc31
            [Summary] => The Linux kernel
            [Arch] => x86_64
        )
)

Name or name-version or name-version-release can be used

Search:

By name

$ rpm -qa \*redis\*
redis-6.0~RC2-1.fc31.remi.x86_64
php-pecl-redis5-5.2.0-1.fc31.remi.7.3.x86_64
php > print_r(rpmdbsearch("*redis*", RPMTAG_NAME, RPMMIRE_GLOB));
Array
(
    [1] => Array
        (
            [Name] => redis
            [Version] => 6.0~RC2
            [Release] => 1.fc31.remi
            [Summary] => A persistent key-value database
            [Arch] => x86_64
        )
    [2] => Array
        (
            [Name] => php-pecl-redis5
            [Version] => 5.2.0
            [Release] => 1.fc31.remi.7.3
            [Summary] => Extension for communicating with the Redis key-value store
            [Arch] => x86_64
        )
)

By installed file

$ rpm -qf $(which php)
php-cli-7.3.15-1.fc31.remi.x86_64
php > print_r(rpmdbsearch(PHP_BINARY, RPMTAG_INSTFILENAMES));
Array
(
    [0] => Array
        (
            [Name] => php-cli
            [Version] => 7.3.15
            [Release] => 1.fc31.remi
            [Summary] => Command-line interface for PHP
            [Arch] => x86_64
        )
)

Dependencies

$ rpm -q --whatprovides php-rpminfo
php-pecl-rpminfo-0.4.0-1.fc31.remi.7.3.x86_64
php > print_r(rpmdbsearch("php-rpminfo", RPMTAG_PROVIDES));
Array
(
    [0] => Array
        (
            [Name] => php-pecl-rpminfo
            [Version] => 0.4.0
            [Release] => 1.fc31.remi.7.3
            [Summary] => RPM information
            [Arch] => x86_64
        )
)

And lof or other criteria, see the RPMTAG_* constants list.

Conclusion

I think that most of the needed search features on RPMs are now available from PHP.

Waiting for review for official Fedora repository, see review #1812855.

You are very welcome to report any bug, or ask for enhancements.