摘要：制定的規范，簡稱，是開發的事實標準。原本有四個規范，分別是自動加載基本代碼規范代碼樣式日志接口年底，新出了第個規范。區別在于的規范比較干凈，去除了兼容以前版本的內容，有一點升級版的感覺。

FIG制定的PHP規范，簡稱PSR，是PHP開發的事實標準。

PSR原本有四個規范，分別是：

PSR-0 自動加載

PSR-1 基本代碼規范

PSR-2 代碼樣式

PSR-3 日志接口

2013年底，新出了第5個規范——PSR-4。

PSR-4規范了如何指定文件路徑從而自動加載類定義，同時規范了自動加載文件的位置。這個乍一看和PSR-0重復了，實際上，在功能上確實有所重復。區別在于PSR-4的規范比較干凈，去除了兼容PHP 5.3以前版本的內容，有一點PSR-0升級版的感覺。當然，PSR-4也不是要完全替代PSR-0，而是在必要的時候補充PSR-0——當然，如果你愿意，PSR-4也可以替代PSR-0。PSR-4可以和包括PSR-0在內的其他自動加載機制共同使用。

PSR-4和PSR-0最大的區別是對下劃線（underscore)的定義不同。PSR-4中，在類名中使用下劃線沒有任何特殊含義。而PSR-0則規定類名中的下劃線_會被轉化成目錄分隔符。

以下代碼展示了遵循PSR-4的類定義，這個類處理多個命名空間：

register();
 *      
 *      // register the base directories for the namespace prefix
 *      $loader->addNamespace("FooBar", "/path/to/packages/foo-bar/src");
 *      $loader->addNamespace("FooBar", "/path/to/packages/foo-bar/tests");
 * 
 * The following line would cause the autoloader to attempt to load the
 * FooBarQuxQuux class from /path/to/packages/foo-bar/src/Qux/Quux.php:
 * 
 *      prefixes[$prefix]) === false) {
            $this->prefixes[$prefix] = array();
        }

        // retain the base directory for the namespace prefix
        if ($prepend) {
            array_unshift($this->prefixes[$prefix], $base_dir);
        } else {
            array_push($this->prefixes[$prefix], $base_dir);
        }
    }

    /**
     * Loads the class file for a given class name.
     *
     * @param string $class The fully-qualified class name.
     * @return mixed The mapped file name on success, or boolean false on
     * failure.
     */
    public function loadClass($class)
    {
        // the current namespace prefix
        $prefix = $class;

        // work backwards through the namespace names of the fully-qualified
        // class name to find a mapped file name
        while (false !== $pos = strrpos($prefix, "")) {

            // retain the trailing namespace separator in the prefix
            $prefix = substr($class, 0, $pos + 1);

            // the rest is the relative class name
            $relative_class = substr($class, $pos + 1);

            // try to load a mapped file for the prefix and relative class
            $mapped_file = $this->loadMappedFile($prefix, $relative_class);
            if ($mapped_file) {
                return $mapped_file;
            }

            // remove the trailing namespace separator for the next iteration
            // of strrpos()
            $prefix = rtrim($prefix, "");   
        }

        // never found a mapped file
        return false;
    }

    /**
     * Load the mapped file for a namespace prefix and relative class.
     * 
     * @param string $prefix The namespace prefix.
     * @param string $relative_class The relative class name.
     * @return mixed Boolean false if no mapped file can be loaded, or the
     * name of the mapped file that was loaded.
     */
    protected function loadMappedFile($prefix, $relative_class)
    {
        // are there any base directories for this namespace prefix?
        if (isset($this->prefixes[$prefix]) === false) {
            return false;
        }

        // look through base directories for this namespace prefix
        foreach ($this->prefixes[$prefix] as $base_dir) {

            // replace the namespace prefix with the base directory,
            // replace namespace separators with directory separators
            // in the relative class name, append with .php
            $file = $base_dir
                  . str_replace("", DIRECTORY_SEPARATOR, $relative_class)
                  . ".php";
            $file = $base_dir
                  . str_replace("", "/", $relative_class)
                  . ".php";

            // if the mapped file exists, require it
            if ($this->requireFile($file)) {
                // yes, we"re done
                return $file;
            }
        }

        // never found it
        return false;
    }

    /**
     * If a file exists, require it from the file system.
     * 
     * @param string $file The file to require.
     * @return bool True if the file exists, false if not.
     */
    protected function requireFile($file)
    {
        if (file_exists($file)) {
            require $file;
            return true;
        }
        return false;
    }
}

相應的單元測試代碼

files = $files;
    }

    protected function requireFile($file)
    {
        return in_array($file, $this->files);
    }
}

class Psr4AutoloaderClassTest extends PHPUnit_Framework_TestCase
{
    protected $loader;

    protected function setUp()
    {
        $this->loader = new MockPsr4AutoloaderClass;

        $this->loader->setFiles(array(
            "/vendor/foo.bar/src/ClassName.php",
            "/vendor/foo.bar/src/DoomClassName.php",
            "/vendor/foo.bar/tests/ClassNameTest.php",
            "/vendor/foo.bardoom/src/ClassName.php",
            "/vendor/foo.bar.baz.dib/src/ClassName.php",
            "/vendor/foo.bar.baz.dib.zim.gir/src/ClassName.php",
        ));

        $this->loader->addNamespace(
            "FooBar",
            "/vendor/foo.bar/src"
        );

        $this->loader->addNamespace(
            "FooBar",
            "/vendor/foo.bar/tests"
        );

        $this->loader->addNamespace(
            "FooBarDoom",
            "/vendor/foo.bardoom/src"
        );

        $this->loader->addNamespace(
            "FooBarBazDib",
            "/vendor/foo.bar.baz.dib/src"
        );

        $this->loader->addNamespace(
            "FooBarBazDibimGir",
            "/vendor/foo.bar.baz.dib.zim.gir/src"
        );
    }

    public function testExistingFile()
    {
        $actual = $this->loader->loadClass("FooBarClassName");
        $expect = "/vendor/foo.bar/src/ClassName.php";
        $this->assertSame($expect, $actual);

        $actual = $this->loader->loadClass("FooBarClassNameTest");
        $expect = "/vendor/foo.bar/tests/ClassNameTest.php";
        $this->assertSame($expect, $actual);
    }

    public function testMissingFile()
    {
        $actual = $this->loader->loadClass("No_VendorNo_PackageNoClass");
        $this->assertFalse($actual);
    }

    public function testDeepFile()
    {
        $actual = $this->loader->loadClass("FooBarBazDibimGirClassName");
        $expect = "/vendor/foo.bar.baz.dib.zim.gir/src/ClassName.php";
        $this->assertSame($expect, $actual);
    }

    public function testConfusion()
    {
        $actual = $this->loader->loadClass("FooBarDoomClassName");
        $expect = "/vendor/foo.bar/src/DoomClassName.php";
        $this->assertSame($expect, $actual);

        $actual = $this->loader->loadClass("FooBarDoomClassName");
        $expect = "/vendor/foo.bardoom/src/ClassName.php";
        $this->assertSame($expect, $actual);
    }
}

PHP的包管理系統Composer已經支持PSR-4，同時也允許在composer.json中定義不同的prefix使用不同的自動加載機制。

Composer使用PSR-0風格

vendor/
    vendor_name/
        package_name/
            src/
                Vendor_Name/
                    Package_Name/
                        ClassName.php       # Vendor_NamePackage_NameClassName
            tests/
                Vendor_Name/
                    Package_Name/
                        ClassNameTest.php   # Vendor_NamePackage_NameClassName

Composer使用PSR-4風格

vendor/
    vendor_name/
        package_name/
            src/
                ClassName.php       # Vendor_NamePackage_NameClassName
            tests/
                ClassNameTest.php   # Vendor_NamePackage_NameClassNameTest

對比以上兩種結構，明顯可以看出PSR-4帶來更簡潔的文件結構。

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