src/PEAR/Dependency2.php
function validatePackage($pkg, &$dl, $params = array())
    {
        if (is_array($pkg) && isset($pkg['info'])) {
            $deps = $this->_dependencydb->getDependentPackageDependencies($pkg['info']);
        } else {
            $deps = $this->_dependencydb->getDependentPackageDependencies($pkg);
        }

        $fail = false;
        if ($deps) {
            if (!class_exists('PEAR_Downloader_Package')) {
                require_once 'PEAR/Downloader/Package.php';
            }

            $dp = &new PEAR_Downloader_Package($dl);
            if (is_object($pkg)) {
                $dp->setPackageFile($pkg);
            } else {
                $dp->setDownloadURL($pkg);
            }

            PEAR::pushErrorHandling(PEAR_ERROR_RETURN);
            foreach ($deps as $channel => $info) {
                foreach ($info as $package => $ds) {
                    foreach ($params as $packd) {
                        if (strtolower($packd->getPackage()) == strtolower($package) &&
                              $packd->getChannel() == $channel) {
                            $dl->log(3, 'skipping installed package check of "' .
                                        $this->_registry->parsedPackageNameToString(
                                            array('channel' => $channel, 'package' => $package),
                                            true) .
                                        '", version "' . $packd->getVersion() . '" will be ' .
                                        'downloaded and installed');
                            continue 2; // jump to next package
                        }
                    }

                    foreach ($ds as $d) {
                        $checker = &new PEAR_Dependency2($this->_config, $this->_options,
                            array('channel' => $channel, 'package' => $package), $this->_state);
                        $dep = $d['dep'];
                        $required = $d['type'] == 'required';
                        $ret = $checker->_validatePackageDownload($dep, $required, array(&$dp));
                        if (is_array($ret)) {
                            $dl->log(0, $ret[0]);
                        } elseif (PEAR::isError($ret)) {
                            $dl->log(0, $ret->getMessage());
                            $fail = true;
                        }
                    }
                }
            }
            PEAR::popErrorHandling();
        }

        if ($fail) {
            return $this->raiseError(
                '%s cannot be installed, conflicts with installed packages');
        }

        return true;
    }
function validatePackage($pkg, &$dl, $params = array())
    {
        if (is_array($pkg) && isset($pkg['info'])) {
            $deps = $this->_dependencydb->getDependentPackageDependencies($pkg['info']);
        } else {
            $deps = $this->_dependencydb->getDependentPackageDependencies($pkg);
        }

        $fail = false;
        if ($deps) {
            if (!class_exists('PEAR_Downloader_Package')) {
                require_once 'PEAR/Downloader/Package.php';
            }

            $dp = &new PEAR_Downloader_Package($dl);
            if (is_object($pkg)) {
                $dp->setPackageFile($pkg);
            } else {
                $dp->setDownloadURL($pkg);
            }

            PEAR::pushErrorHandling(PEAR_ERROR_RETURN);
            foreach ($deps as $channel => $info) {
                foreach ($info as $package => $ds) {
                    foreach ($params as $packd) {
                        if (strtolower($packd->getPackage()) == strtolower($package) &&
                              $packd->getChannel() == $channel) {
                            $dl->log(3, 'skipping installed package check of "' .
                                        $this->_registry->parsedPackageNameToString(
                                            array('channel' => $channel, 'package' => $package),
                                            true) .
                                        '", version "' . $packd->getVersion() . '" will be ' .
                                        'downloaded and installed');
                            continue 2; // jump to next package
                        }
                    }

                    foreach ($ds as $d) {
                        $checker = &new PEAR_Dependency2($this->_config, $this->_options,
                            array('channel' => $channel, 'package' => $package), $this->_state);
                        $dep = $d['dep'];
                        $required = $d['type'] == 'required';
                        $ret = $checker->_validatePackageDownload($dep, $required, array(&$dp));
                        if (is_array($ret)) {
                            $dl->log(0, $ret[0]);
                        } elseif (PEAR::isError($ret)) {
                            $dl->log(0, $ret->getMessage());
                            $fail = true;
                        }
                    }
                }
            }
            PEAR::popErrorHandling();
        }

        if ($fail) {
            return $this->raiseError(
                '%s cannot be installed, conflicts with installed packages');
        }

        return true;
    }
validate a downloaded package against installed packages

As of PEAR 1.4.3, this will only validate

@param array|PEAR_Downloader_Package|PEAR_PackageFile_v1|PEAR_PackageFile_v2
$pkg package identifier (either
array('package' => blah, 'channel' => blah) or an array with
index 'info' referencing an object)
@param PEAR_Downloader $dl
@param array $params full list of packages to install
@return true|PEAR_Error