I've finally found a way to verify signature. Sample in the documentation doesn't work. Code bellow DOES work :)

<?php
// $data is assumed to contain the data to be signed

// fetch certificate from file and ready it
$fp = fopen("path/file.pem", "r");
$cert = fread($fp, 8192);
fclose($fp);

// state whether signature is okay or not
// use the certificate, not the public key
$ok = openssl_verify($data, $signature, $cert);
if ($ok == 1) {
    echo "good";
} elseif ($ok == 0) {
    echo "bad";
} else {
    echo "ugly, error checking signature";
}
?>

I spent days scouring the php openssl documentation trying to figure out how to do what sounds like a simple task - given two PEM encoded certificates, is one the signer of the other?  Nowhere in the openssl_verify() documentation or comments is it explained where to obtain the signature of an existing certificate.  The openssl_x509_parse() function looked promising, but it is an unstable API that may change.

I had to write my own code to determine if one cert signed another, it is located here: http://badpenguins.com/source/misc/isCertSigner.php?viewSource

In a nutshell here is what I learned...

The signature data in a signed X.509 certificate contains DER formatted data about the signature that is encrypted with the signers public key.  The data contains a hash of the original subject certificate and information about what encryption algorithm was used to create the signature.

So you need to get this signature data and a copy of the original certificate with the issuer and signature sequences removed.  Hash a copy of the original certificate (sans issuer/signature sequences) with the same algorithm the issuer used and if the hashes match, you have the issuer cert that signed the certificate.

Anbybody trying to get a Win32 CryptoAPI based digital signature component to work with the openssl_verify() function should be aware that the CryptoAPI PKCS1 (RSA) method uses bytes in reverse order while the openssl_verify() method expects a correctly formatted PKCS1 digital signature (as should be). I learned this the hard way and it took me some time to dig this out. A simple solution in VBScript to reverse the byte order:

N = Len(Blob.Hex)

' reverse bytes in the signature using Hex format 
For i = 1 To N - 1 Step 2 
    s = Mid(Blob, i, 2) & s 
Next 

s contains the digital signature in reverse order. Blob is an arbitrary binary container.

Send the signature off in Hex format and use a hex2bin method in PHP to convert to the correct format for openssl_verify(), i.e.

function hex2bin($data) {

    $len = strlen($data);
    return pack("H" . $len, $data);

} 

That's it, hope it helps out. BTW I used ASPEncrypt to toy around with on Win32 platform. Works only with Internet Explorer but you could also use a Java applet and have none of the abovementioned problems :-)

openssl_verify() is populating openssl_error_string() even on false. 

When openssl_verify() returns 0, openssl_error_string() is populated with 1.
I spent lot of time to understand, while my next call to openssl was failing with checks for error.

<?php
$c = file_get_contents($filename);
$publicKey = openssl_pkey_get_public($c);
$result = openssl_verify('freedom', 'someirrelevantnosign', $publicKey);

$error = "";

while ($msg = openssl_error_string() !== false) {
    $error .= $msg;
}

if (!empty($error)) {
    echo $error; // 1
}

A note about the openssl_verify() (and some of the other functions).  The public key comes from a certificate in any of the support formats (as the example shows, use openssl_get_publickey() to get the resource id).  But after some trial and error I found the signature string MUST BE BINARY.  While no error occurs, passing a base64-formatted signature string (PEM format?), you simply get a mismatch.  When I did the base64 decode myself, the verify returned a match (return value 1).  You can simply drop the begin/end lines and take the output of the 'base64_decode()' function.

As stated from the doc: "Returns 1 if the signature is correct, 0 if it is incorrect, and -1 or false on error. "

In the second example as a well as in Stiv's note, following condition will match for both 0 or false, which have different meaning:
elseif ($ok == 0) {
    echo "bad";
}

On should do an identical test here (===) instead of an equal test (==):
elseif ($ok === 0) {
    echo "bad";
}
---
var_dump(0==false); //==> true
var_dump(0===false);//==> false

PHP is the most efficient language for handling web requests. I have tested that PHP's OpenSSL library can decrypt data encrypted by JS using AES, OAEP, and EC-DH, and can also verify data signed by JS using HMAC and RSA-PKCS#1.5. However, it cannot verify data signed by JS using EC-DSA or RSA-PSS. Verifying data signed with EC-DSA and RSA-PSS is necessary. However, PHP's OpenSSL library still does not support verifying EC-DSA and RSA-PSS signatures.

// JS EC-DSA sign
crypto.subtle.generateKey(
    {
        name: 'ECDSA',
        namedCurve: 'P-256'    // P-521 corresponds to secp521r1, P-384 corresponds to secp384r1, p-256 corresponds to prime256v1
    },
    true,
    ['sign', 'verify']
).then(pair => {
    return {
        publicKey: crypto.subtle.exportKey('spki', pair.publicKey),
        signature: crypto.subtle.sign(
        {
            name: 'ECDSA',
            hash: {
                name: 'SHA-1' // or SHA-256, SHA-384, SHA-512
            }
        },
            pair.privateKey,
            (new TextEncoder()).encode('test data')
        )
    };
}).then(data => {
    // await crypto.subtle.verify(...) is true;
    // convert data.publicKey ArrayBuffer to PEM publicKey and encodeURIComponent it;
    // convert data.signature ArrayBuffer to base64 string;
    // send data object to PHP
});
// PHP receives the above data and executes openssl_verify
openssl_verify(
    base64_decode($_POST['signature']),
    'test data',
    urldecode($_POST['publicKey']),
    OPENSSL_ALGO_SHA1 // or OPENSSL_ALGO_SHA256, OPENSSL_ALGO_SHA384, OPENSSL_ALGO_SHA512
);
// result: -1

// JS RSA-PSS sign
crypto.subtle.generateKey(
    {
        name: 'RSA-PSS',
        modulusLength: 2048,
        publicExponent: new Uint8Array([1, 0, 1]),
        hash: 'SHA-256' // or SHA-1, SHA-384, SHA-512
    },
    true,
    ['sign', 'verify']
).then(pair => {
    return {
        publicKey: crypto.subtle.exportKey('spki', pair.publicKey),
        signature: crypto.subtle.sign(
            {
                name: 'RSA-PSS',
                saltLength: 32
            },
            pair.privateKey,
            (new TextEncoder()).encode('test data')
        )
    };
}).then(data => {
    // await crypto.subtle.verify(...) is true;
    // convert data.publicKey ArrayBuffer to PEM publicKey and encodeURIComponent it;
    // convert data.signature ArrayBuffer to base64 string;
    // send data object to PHP
});

// PHP receives the above data and executes openssl_verify
openssl_verify(
    base64_decode($_POST['signature']),
    'test data',
    urldecode($_POST['publicKey']),
    OPENSSL_ALGO_SHA256 // or OPENSSL_ALGO_SHA1, OPENSSL_ALGO_SHA384, OPENSSL_ALGO_SHA512
);
// result: -1, but RSA-PKCS#1.5 is OK

mikey at badpenguins dot com -- validating an X509 certificate chain in php seems to be possible with openssl_x509_checkpurpose()

You can actually use the public key as third parameter and not the certificate.

If you can't make it work, make sure that :

1) Your public key is well formatted. It seems that it must have the ----BEGIN PUBLIC KEY---- and ----END PUBLIC KEY----

2) Your signature is in binary format. You can use the php base64_decode for this.