Can't Call Functions in PHP Attribute Defaults
[Problem]
Despite having previous programming experience, a novice in PHP is perplexed by an attribute default error. The code:
<code class="php">class Foo { public $path = array( realpath(".") ); }</code>
yields a syntax error. However, the following works seamlessly:
<code class="php">$path = array( realpath(".") );</code>
The question arises: why can't functions be invoked in attribute defaults? Is this intentional or a flaw in implementation?
[Answer]
The PHP compiler code indicates this restriction is intentional, although no official rationale is available. Implementing this functionality reliably poses certain challenges, as evidenced by limitations in PHP's current implementation.
The compiler's grammar defines a class variable declaration as:
class_variable_declaration: //... | T_VARIABLE '=' static_scalar //... ;
Therefore, to assign variable values like $path, the expected value must align with a static scalar. This encompasses arrays with values that are also static scalars:
static_scalar: /* compile-time evaluated scalars */ //... | T_ARRAY '(' static_array_pair_list ')' // ... //... ;
If the grammar allowed the following syntax, which aligns with the code sample, the script would encounter a "Invalid binding type" error:
class_variable_declaration: //... | T_VARIABLE '=' T_ARRAY '(' array_pair_list ')' // ... ;
Parsing the given code sample reveals the following steps:
zend_do_begin_class_declaration() // Adds an opcode array_init(), zend_do_add_static_array_element() // Do not create new opcodes, add array to class properties zend_do_declare_property() // Declares the property zend_do_early_binding() // Consumes the last opcode and evaluates it
If the opcode is not expected (e.g., related to functions or methods), an error is thrown.
Allowing non-static arrays generates an INIT_ARRAY opcode, which disrupts zend_do_early_binding():
DECLARE_CLASS 'Foo' SEND_VAL '.' DO_FCALL 'realpath' INIT_ARRAY
To accommodate function calls in attribute defaults, a new opcode array scoped to the class variable declaration would be needed, similar to method definitions. However, determining the timing of such evaluation presents additional challenges.
Other dynamic languages have managed to resolve this, but it remains a feature absent in PHP, potentially due to its complexity and perceived low priority.
The above is the detailed content of Why Can\'t Attribute Defaults Function Calls in PHP?. For more information, please follow other related articles on the PHP Chinese website!

Hot AI Tools

Undress AI Tool
Undress images for free

Undresser.AI Undress
AI-powered app for creating realistic nude photos

AI Clothes Remover
Online AI tool for removing clothes from photos.

Clothoff.io
AI clothes remover

Video Face Swap
Swap faces in any video effortlessly with our completely free AI face swap tool!

Hot Article

Hot Tools

Notepad++7.3.1
Easy-to-use and free code editor

SublimeText3 Chinese version
Chinese version, very easy to use

Zend Studio 13.0.1
Powerful PHP integrated development environment

Dreamweaver CS6
Visual web development tools

SublimeText3 Mac version
God-level code editing software (SublimeText3)

Hot Topics

TosecurelyhandleauthenticationandauthorizationinPHP,followthesesteps:1.Alwayshashpasswordswithpassword_hash()andverifyusingpassword_verify(),usepreparedstatementstopreventSQLinjection,andstoreuserdatain$_SESSIONafterlogin.2.Implementrole-basedaccessc

To safely handle file uploads in PHP, the core is to verify file types, rename files, and restrict permissions. 1. Use finfo_file() to check the real MIME type, and only specific types such as image/jpeg are allowed; 2. Use uniqid() to generate random file names and store them in non-Web root directory; 3. Limit file size through php.ini and HTML forms, and set directory permissions to 0755; 4. Use ClamAV to scan malware to enhance security. These steps effectively prevent security vulnerabilities and ensure that the file upload process is safe and reliable.

In PHP, the main difference between == and == is the strictness of type checking. ==Type conversion will be performed before comparison, for example, 5=="5" returns true, and ===Request that the value and type are the same before true will be returned, for example, 5==="5" returns false. In usage scenarios, === is more secure and should be used first, and == is only used when type conversion is required.

The methods of using basic mathematical operations in PHP are as follows: 1. Addition signs support integers and floating-point numbers, and can also be used for variables. String numbers will be automatically converted but not recommended to dependencies; 2. Subtraction signs use - signs, variables are the same, and type conversion is also applicable; 3. Multiplication signs use * signs, which are suitable for numbers and similar strings; 4. Division uses / signs, which need to avoid dividing by zero, and note that the result may be floating-point numbers; 5. Taking the modulus signs can be used to judge odd and even numbers, and when processing negative numbers, the remainder signs are consistent with the dividend. The key to using these operators correctly is to ensure that the data types are clear and the boundary situation is handled well.

Yes, PHP can interact with NoSQL databases like MongoDB and Redis through specific extensions or libraries. First, use the MongoDBPHP driver (installed through PECL or Composer) to create client instances and operate databases and collections, supporting insertion, query, aggregation and other operations; second, use the Predis library or phpredis extension to connect to Redis, perform key-value settings and acquisitions, and recommend phpredis for high-performance scenarios, while Predis is convenient for rapid deployment; both are suitable for production environments and are well-documented.

TostaycurrentwithPHPdevelopmentsandbestpractices,followkeynewssourceslikePHP.netandPHPWeekly,engagewithcommunitiesonforumsandconferences,keeptoolingupdatedandgraduallyadoptnewfeatures,andreadorcontributetoopensourceprojects.First,followreliablesource

PHPbecamepopularforwebdevelopmentduetoitseaseoflearning,seamlessintegrationwithHTML,widespreadhostingsupport,andalargeecosystemincludingframeworkslikeLaravelandCMSplatformslikeWordPress.Itexcelsinhandlingformsubmissions,managingusersessions,interacti

TosettherighttimezoneinPHP,usedate_default_timezone_set()functionatthestartofyourscriptwithavalididentifiersuchas'America/New_York'.1.Usedate_default_timezone_set()beforeanydate/timefunctions.2.Alternatively,configurethephp.inifilebysettingdate.timez
