MDL-64723 tool_mobile: workaround for invalid certificate parsing.

This is to account for specific server configuration that are affected
by one of the following issues, which results in certificate signature
algorithms being incorrectly parsed:

 * https://bugs.php.net/bug.php?id=77548
 * curl/curl#3706

admin/tool/mobile/classes/api.php

@@ -603,8 +603,21 @@ public static function get_potential_config_issues() {
                 $timenow = time();
                 $expectedissuer = null;
                 foreach ($info['certinfo'] as $cert) {
+
+                    // Due to a bug in certain curl/openssl versions the signature algorithm isn't always correctly parsed.
+                    // See https://github.com/curl/curl/issues/3706 for reference.
+                    if (!array_key_exists('Signature Algorithm', $cert)) {
+                        // The malformed field that does contain the algorithm we're looking for looks like the following:
+                        // <WHITESPACE>Signature Algorithm: <ALGORITHM><CRLF><ALGORITHM>.
+                        preg_match('/\s+Signature Algorithm: (?<algorithm>[^\s]+)/', $cert['Public Key Algorithm'], $matches);
+
+                        $signaturealgorithm = $matches['algorithm'] ?? '';
+                    } else {
+                        $signaturealgorithm = $cert['Signature Algorithm'];
+                    }
+
                     // Check if the signature algorithm is weak (Android won't work with SHA-1).
-                    if ($cert['Signature Algorithm'] == 'sha1WithRSAEncryption' || $cert['Signature Algorithm'] == 'sha1WithRSA') {
+                    if ($signaturealgorithm == 'sha1WithRSAEncryption' || $signaturealgorithm == 'sha1WithRSA') {
                         $warnings[] = ['insecurealgorithmwarning', 'tool_mobile'];
                     }
                     // Check certificate start date.