php script fails to connect to MySQL with SSL

The script below runs on a Centos server and is trying to connect to a MySQL database on another server which requires SSL parameters. The credentials used in the script work fine using and Microsoft Access DSN connection.

<?php
ini_set ('error_reporting', E_ALL);
ini_set ('display_errors', '1');
error_reporting (E_ALL|E_STRICT);

$pdo = new PDO('mysql:host=99.99.199.199;dbname=dummy1', 'user1', 'pwd1', 
array(
    PDO::MYSQL_ATTR_SSL_KEY    =>'/etc/mysql/ssl/ck.pem',
    PDO::MYSQL_ATTR_SSL_CERT=>'/etc/mysql/ssl/cc.pem',
    PDO::MYSQL_ATTR_SSL_CA    =>'/etc/mysql/ssl/c1.pem'
));
$statement = $pdo->query("SHOW TABLES;");
$row = $statement->fetch(PDO::FETCH_ASSOC);
echo htmlentities($row['_message']);
?>

The code above gives SSL operation failed with code 1 - here is the full message:

Fatal error: Uncaught PDOException: PDO::__construct(): SSL operation failed with code 1. OpenSSL Error messages: error:14090086:SSL routines:ssl3_get_server_certificate:certificate verify failed in /var/www/vhosts/zzzzz.org/httpdocs/zzodbc/dgodbc1.php:10 Stack trace: #0 /var/www/vhosts/zzzzz.org/httpdocs/zzodbc/dgodbc1.php(10): PDO->__construct('mysql:host=99.9...', 'odbc_guil...', 'pwd1', Array) #1 {main} Next PDOException: SQLSTATE[HY000] [2002] in /var/www/vhosts/zzzzz.org/httpdocs/zzodbc/dgodbc1.php:10 Stack trace: #0 /var/www/vhosts/zzzzz.org/httpdocs/zzodbc/dgodbc1.php(10): PDO->__construct('mysql:host=99.9...', 'odbc_guil...', 'pwd1', Array) #1 {main} thrown in /var/www/vhosts/zzzzz.org/httpdocs/zzodbc/dgodbc1.php on line 10

I have verified that the credentials, including the SSL parameters with a DSN connection. I have checked that the SSL Keys are correctly located in the /etc/mysql/ssl directory.

Any help to suggest what I'm doing wrong would be good. Thanks.

I may have been going at this in the wrong way.... Since these keys work with ODBC then I think I should be using using odbc_connect and sending the same string as I use with MS access such as

$user = "user";
$pass = "pwd";

$connection = "Driver={MySQL ODBC 5.1 Driver};Server=46.51.178.163;Database=db1;sslca=/etc/mysql/ssl/c1.pem;sslkey=/etc/mysql/ssl/ck.pem;sslcapath=/etc/mysql/ssl/;sslcert=/etc/mysql/ssl/cc.pem";

$con = odbc_connect($connection, $user, $pass);

But to get this to work I need to install a MySQL connector on the server which I'm grappling with at the moment.

I have solved this problem -thanks for all who have helped. This is what I have learned:

SSL keys are connection type specific - so I had keys that worked with ODBC and it was wrong to expect them to work with PDO
ODBC drivers ( php extensions ) need to be installed on the server - they aren't automatically present. Here is an excellent video showing how to do this.
You need command line access to the server to install the driver ( and also to upload the SSL keys to a secure location ) - they are in /etc/mysql/ssl.
I installed the driver in /usr/lib/odbc2/lib rather than in the long folder name in the video. I also installed the in the /usr tree because when I tried the locations in the video I got file not found errors. The two driver files are libmyodbc5a.so and libodbc5w.so. Only the ...5w.so file seems to be required.
Once these files are in place then you need to add an entry to odbcinst.ini in the /etc folder. I used nano so the command line nano odbcinst.ini brings up the file which had a model entry for PostgresSQL. If the server is 64 bit then these are the entries I made in odbcinst.ini: [mysql537] Driver64 = /usr/lib/odbc2/lib/libmyodbc5w.so Setup64 = /usr/lib/odbc2/lib/libmyodbc5w.so UsageCount = 1
You must have the ...64 paths otherwise the driver isn't found ( i.e Driver64 = NOT Driver= ). I made this mistake first off.
Provided the driver files are found at the paths in odbcinst.ini then things should work. (I thought I needed entries in odbc.ini but I now believe you only need something here if you are using a DSN).
the folder odbc2 was one I created inside /etc/lib which already exists. I did that to avoid any permission issues by creating a new folder.

Here is the code that works ( the connection string is exactly the same as the string used in a Microsoft Access connection ):

<?php
ini_set ('error_reporting', E_ALL);
ini_set ('display_errors', '1');
error_reporting (E_ALL|E_STRICT);

$user = "odbcmmm";
$pass = "999999999";

$connection = "Driver={mysql537};Server=99.99.199.199;Database=db_name;UID=odbc_db_name;PWD=password;sslca=/etc/mysql/ssl/c1.pem;sslkey=/etc/mysql/ssl/ck.pem;sslcapath=/etc/mysql/ssl/;sslcert=/etc/mysql/ssl/cc.pem";

$con = odbc_connect($connection, $user, $pass);

$sql="SELECT Id from stk_item"; 
$rs=odbc_exec($con,$sql);

if (!$rs)   {
exit("Error in SQL");
}

I hope this is useful.