There is something like a coding standard for Magento 1, called ECG where numerous things should be avoided or methods that should not be used.

Let's try to create a list that covers all cases that are throwing "errors" or "warning" in your code.

Below you'll find a list with all possible warnings. I'll update this post at regular intervals and link to given and upvoted answers.

Note: please try to avoid duplicate answers ;)

Download: https://github.com/magento-ecg/coding-standard

ECG Sniffs

Classes

Mysql4

Mysql4 classes are obsolete.

• How to handle deprecated “Mysql4” classes when rewriting resource models


• Magento 1.9 module oddly breaking when changing resource folder from Mysql4 to Resource

Object Instantiation

Direct object instantiation (class %s) is discouraged in Magento.

protected $disallowedClassPrefixes = array(

    'Mage_',

    'Enterprise_',

);

...

PHP

Goto

Use of goto is discouraged.

...

Namespace

Namespace for "'.$exceptionClassName.'" class is not specified.

...

Private Class Member

Private class member detected.

...

Var

Private class member detected.

...

Performance

Collection Count

Unnecessary loading of a Magento data collection. Use the getSize() method instead.

• How to check if a collection has items?

FetchAll

fetchAll() can be memory inefficient for large data sets.

...

GetFirstItem

getFirstItem() does not limit the result of collection load to one item.

• GetFirstItem - https://magento.stackexchange.com/a/179309/46249
  

Loop

Array size calculation function %s detected in loop

Model LSD method %s detected in loop

Data load %s method detected in loop

protected $countFunctions = array(

    'sizeof',

    'count'

);

protected $modelLsdMethods = array(

    'load',

    'save',

    'delete'

);

• load
  
  
  
  
  • Find MIN & MAX value of product attribute in a large category
  
  
  
  


• ...

Security

Acl

Missing the %s() ACL method in the %s class.

const PARENT_CLASS_NAME = 'Mage_Adminhtml_Controller_Action';

const REQUIRED_ACL_METHOD_NAME = '_isAllowed';

...

Discouraged Function

public $forbiddenFunctions = array(

    '^is_dir' => null,

    '^is_file$' => null,

    '^pathinfo$' => null,

);

...

Forbidden Function

public $forbiddenFunctions = array(

    '^assert$' => null,

    '^bind_textdomain_codeset$' => null,

    '^bindtextdomain$' => null,

    '^bz.*$' => null,

    '^call_user_func$' => null,

    '^call_user_func_array$' => null,

    '^chdir$' => null,

    '^chgrp$' => null,

    '^chmod$' => null,

    '^chown$' => null,

    '^chroot$' => null,

    '^com_load_typelib$' => null,

    '^copy$' => null,

    '^create_function$' => null,

    '^curl_.*$' => null,

    '^cyrus_connect$' => null,

    '^dba_.*$' => null,

    '^dbase_.*$' => null,

    '^dbx_.*$' => null,

    '^dcgettext$' => null,

    '^dcngettext$' => null,

    '^dgettext$' => null,

    '^dio_.*$' => null,

    '^dirname$' => null,

    '^dngettext$' => null,

    '^domxml_.*$' => null,

    '^exec$' => null,

    '^fbsql_.*$' => null,

    '^fdf_add_doc_javascript$' => null,

    '^fdf_open$' => null,

    '^fopen$' => null,

    '^fsockopen$' => null,

    '^ftp_.*$' => null,

    '^fwrite$' => null,

    '^gettext$' => null,

    '^gz.*$' => null,

    '^header$' => null,

    '^highlight_file$' => null,

    '^ibase_.*$' => null,

    '^id3_set_tag$' => null,

    '^ifx_.*$' => null,

    '^image.*$' => null,

    '^imap_.*$' => null,

    '^ingres_.*$' => null,

    '^ircg_.*$' => null,

    '^ldap_.*$' => null,

    '^link$' => null,

    '^mail$' => null,

    '^mb_send_mail$' => null,

    '^mkdir$' => null,

    '^move_uploaded_file$' => null,

    '^msession_.*$' => null,

    '^msg_send$' => null,

    '^msql$' => null,

    '^msql_.*$' => null,

    '^mssql_.*$' => null,

    '^mysql_.*$' => null,

    '^odbc_.*$' => null,

    '^opendir$' => null,

    '^openlog$' => null,

    '^ora_.*$' => null,

    '^ovrimos_.*$' => null,

    '^parse_ini_file$' => null,

    '^parse_str$' => null,

    '^parse_url$' => null,

    '^parsekit_compile_string$' => null,

    '^passthru$' => null,

    '^pcntl_.*$' => null,

    '^posix_.*$' => null,

    '^pfpro_.*$' => null,

    '^pfsockopen$' => null,

    '^pg_.*$' => null,

    '^php_check_syntax$' => null,

    '^popen$' => null,

    '^print_r$' => null,

    '^printf$' => null,

    '^proc_open$' => null,

    '^putenv$' => null,

    '^readfile$' => null,

    '^readgzfile$' => null,

    '^readline$' => null,

    '^readlink$' => null,

    '^register_shutdown_function$' => null,

    '^register_tick_function$' => null,

    '^rename$' => null,

    '^rmdir$' => null,

    '^scandir$' => null,

    '^session_.*$' => null,

    '^set_include_path$' => null,

    '^set_ini$' => null,

    '^set_time_limit$' => null,

    '^setcookie$' => null,

    '^setlocale$' => null,

    '^setrawcookie$' => null,

    '^shell_exec$' => null,

    '^sleep$' => null,

    '^socket_.*$' => null,

    '^stream_.*$' => null,

    '^sybase_.*$' => null,

    '^symlink$' => null,

    '^syslog$' => null,

    '^system$' => null,

    '^touch$' => null,

    '^trigger_error$' => null,

    '^unlink$' => null,

    '^vprintf$' => null,

    '^mysqli.*$' => null,

    '^oci_connect$' => null,

    '^oci_pconnect$' => null,

    '^quotemeta$' => null,

    '^sqlite_popen$' => null,

    '^time_nanosleep$' => null,

    '^base64_decode$' => null,

    '^base_convert$' => null,

    '^basename$' => null,

    '^chr$' => null,

    '^convert_cyr_string$' => null,

    '^dba_nextkey$' => null,

    '^dns_get_record$' => null,

    '^extract$' => null,

    '^fdf_.*$' => null,

    '^fget.*$' => null,

    '^fread$' => null,

    '^fflush$' => null,

    '^get_browser$' => null,

    '^get_headers$' => null,

    '^get_meta_tags$' => null,

    '^getallheaders$' => null,

    '^getenv$' => null,

    '^getopt$' => null,

    '^headers_list$' => null,

    '^hebrev$' => null,

    '^hebrevc$' => null,

    '^highlight_string$' => null,

    '^html_entity_decode$' => null,

    '^ibase_blob_import$' => null,

    '^id3_get_tag$' => null,

    '^import_request_variables$' => null,

    '^ircg_nickname_unescape$' => null,

    '^ldap_get_values$' => null,

    '^mb_decode_mimeheader$' => null,

    '^mb_parse_str$' => null,

    '^mcrypt_decrypt$' => null,

    '^mdecrypt_generic$' => null,

    '^msg_receive$' => null,

    '^ngettext$' => null,

    '^ob_get_contents$' => null,

    '^ob_get_flush$' => null,

    '^rawurldecode$' => null,

    '^shm_get_var$' => null,

    '^stripcslashes$' => null,

    '^stripslashes$' => null,

    '^token_get_all$' => null,

    '^unpack$' => null,

    '^convert_uudecode$' => null,

    '^iconv_mime_decode$' => null,

    '^iconv_mime_decode_headers$' => null,

    '^iconv_mime_encode$' => null,

    '^iconv_set_encoding$' => null,

    '^php_strip_whitespace$' => null,

    '^addcslashes$' => null,

    '^addslashes$' => null,

    '^escapeshellarg$' => null,

    '^escapeshellcmd$' => null,

    '^gettype$' => null,

    '^var_dump$' => null,

    '^tempnam$' => null,

    '^realpath$' => null,

    '^linkinfo$' => null,

    '^lstat$' => null,

    '^stat$' => null,

    '^lchgrp$' => null,

    '^lchown$' => null,

    '^show_source$' => null,

    '^is_executable$' => null,

    '^is_link$' => null,

    '^is_readable$' => null,

    '^is_writable$' => null,

    '^is_writeable$' => null,

    '^is_uploaded_file$' => null,

    '^glob$' => null,

    '^ssh2_.*$' => null,

    '^delete$' => null,

    '^file.*$' => null,

);

• ...


• curl_: - https://magento.stackexchange.com/a/178642/46249
  


• file_exists - https://magento.stackexchange.com/a/178641/46249
  


• ...

Include File

"%s" statement detected. File manipulations are discouraged.

... Statement is not a function, no parentheses are required.

... Passing urls is forbidden.

... Concatenating is forbidden.

... Variables inside are insecure.

public $urlPattern = '#(https?|ftp)://.*#i';

...

Language Construct

Incorrect usage of back quote string constant. Back quotes should be always inside strings.

Use of %s language construct is discouraged.

    return array(

        T_EXIT,

        T_ECHO,

        T_PRINT,

        T_BACKTICK

    );

...

Superglobal

Direct use of %s Superglobal detected.

public $superGlobalErrors = array(

    '$GLOBALS',

    '$_GET',

    '$_POST',

    '$_SESSION',

    '$_REQUEST',

    '$_ENV'

);

public $superGlobalWarning = array(

    '$_FILES',

    '$_COOKIE',

    '$_SERVER',

);

Sql

Raw Query

Possible raw SQL statement %s detected

public $statements = array(

    'SELECT',

    'UPDATE',

    'INSERT',

    'CREATE',

    'DELETE',

    'ALTER',

    'DROP'

);

public $queryFunctions = array(

    'query',

    'raw_query'

);

...

Slow Query

Possible slow SQL statement %s detected

Possible slow SQL method %s detected

public $adapterMethods = array(

    'group',

    'having',

    'distinct',

    'addLikeEscape',

    'escapeLikeValue',

    'union',

    'orHaving',

);

public $rawStatements = array(

    'GROUP BY',

    'HAVING',

    'DISTINCT',

    'LIKE',

    'UNION',

);

...

Strings

RegEx

Possible executable regular expression in %s. Make sure that the pattern doesn't contain "e" modifier

public $functions = array(

    'preg_replace',

);

...

String Concat

Use of + operator to concatenate two strings detected

...

String Position

Identical operator === is not used for testing the return value of %s function

public $functions = array(

    'strpos',

    'stripos',

);

• http://php.net/manual/en/function.strpos.php#refsect1-function.strpos-examples


• http://php.net/manual/en/function.stripos.php#refsect1-function.stripos-examples

Related questions & answers for best-practice

• What is faster - getting raw attribute value or use collection?


• Magento 1: Performance optimizations to delete entities

Forbidden Function

file_exists()

The use of function file_exists() is forbidden

incorrect:

if (!file_exists($filePath)) {

    ...

}

correct:

$io = new Varien_Io_File();

if (!$io->fileExists($filePath)) {

    ...

}

or

$validatorNot = new Zend_Validate_File_NotExists($path);

if ($validatorNot->isValid($file)) {

    ...

}

GetFirstItem

getFirstItem() does not limit the result of collection load to one item.

incorrect:

$collection = Mage::getModel('catalog/category')

    ->load(41)

    ->getProductCollection()

    ->addAttributeToSelect('weight');



$product = $collection->getFirstItem();

$weight  = $product->getData('weight');

correct:

Apply limit before grab data.

$collection->getSelect()->limit(1)

or

$collection->setPageSize(1, 1)

Expamples:

Collection with 750 products ...

Without limiting before:

• Total Incl. Wall Time (microsec): 2,116,522 microsecs


• Total Incl. CPU (microsecs): 2,101,688 microsecs


• Total Incl. MemUse (bytes): 4,783,504 bytes


• Total Incl. PeakMemUse (bytes): 4,363,112 bytes


• Number of Function Calls: 104,187

With using getSelect()->limit(1):

• Total Incl. Wall Time (microsec): 149,803 microsecs


• Total Incl. CPU (microsecs): 131,405 microsecs


• Total Incl. MemUse (bytes): 2,384,840 bytes


• Total Incl. PeakMemUse (bytes): 1,827,112 bytes


• Number of Function Calls: 5,327

With using setPageSize(1, 1)

• Total Incl. Wall Time (microsec): 155,025 microsecs


• Total Incl. CPU (microsecs): 136,191 microsecs


• Total Incl. MemUse (bytes): 2,413,128 bytes


• Total Incl. PeakMemUse (bytes): 1,856,064 bytes


• Number of Function Calls: 5,515

Note:

This warning will still pop up, even if you limit your collection before. To get rid of this message use $collection->getLastItem() instead.