Information may be passed to functions via the argument list, which is a comma-delimited list of expressions. The arguments are evaluated from left to right.

PHP supports passing arguments by value the default , passing by reference , and default argument values. Variable-length argument lists are also supported. Example 1 Passing arrays to functions. By default, function arguments are passed by value so that if the value of the argument within the function is changed, it does not get changed outside of the function. To allow a function to modify its arguments, they must be passed by reference.

Example 2 Passing function parameters by reference. Example 3 Use of default parameters in functions. PHP also allows the use of array s and the special type NULL as default values, for example:.

Example 4 Using non-scalar types as default values. The default value must be a constant expression, not for example a variable, a class member or a function call. Note that when using default arguments, any defaults should be on the right side of any non-default arguments; otherwise, things will not work as expected.

Consider the following code snippet:. Example 5 Incorrect usage of default function arguments. Example 6 Correct usage of default function arguments.

As of PHP 5, arguments that are passed by reference may have a default value. Type declarations were also known as type hints in PHP 5. Type declarations allow functions to require that parameters are of a certain type at call time.

If the given value is of the incorrect type, then an error is generated: To specify a type declaration, the type name should be added before the parameter name. The declaration can be made to accept NULL values if the default value of the parameter is set to NULL. Aliases for the above scalar types are not supported.

Instead, they are treated as class or interface names. For example, using boolean as a parameter or return type will require an argument or return value that is an instanceof the class or interface boolean , rather than of type bool:. By default, PHP will coerce values of the wrong type into the expected scalar type if possible. For example, a function that is given an integer for a parameter that expects a string will get a variable of type string.

It is possible to enable strict mode on a per-file basis. In strict mode, only a variable of exact type of the type declaration will be accepted, or a TypeError will be thrown. The only exception to this rule is that an integer may be given to a function expecting a float.

Enabling strict mode will also affect return type declarations. Strict typing applies to function calls made from within the file with strict typing enabled, not to the functions declared within that file. If a file without strict typing enabled makes a call to a function that was defined in a file with strict typing, the caller's preference weak typing will be respected, and the value will be coerced.

Strict typing is only defined for scalar type declarations, and as such, requires PHP 7. Example 12 Catching TypeError. PHP has support for variable-length argument lists in user-defined functions.

This is implemented using the The arguments will be passed into the given variable as an array; for example: You can also use You may specify normal positional arguments before the In this case, only the trailing arguments that don't match a positional argument will be added to the array generated by It is also possible to add a type hint before the If this is present, then all arguments captured by Example 15 Type hinted variable arguments.

Finally, you may also pass variable arguments by reference by prefixing the The first example above would be implemented as follows in PHP 5. Example 16 Accessing variable arguments in PHP 5. Getting Started Introduction A simple tutorial Language Reference Basic syntax Types Variables Constants Expressions Operators Control Structures Functions Classes and Objects Namespaces Errors Exceptions Generators References Explained Predefined Variables Predefined Exceptions Predefined Interfaces and Classes Context options and parameters Supported Protocols and Wrappers Security Introduction General considerations Installed as CGI binary Installed as an Apache module Session Security Filesystem Security Database Security Error Reporting Using Register Globals User Submitted Data Magic Quotes Hiding PHP Keeping Current Features HTTP authentication with PHP Cookies Sessions Dealing with XForms Handling file uploads Using remote files Connection handling Persistent Database Connections Safe Mode Command line usage Garbage Collection DTrace Dynamic Tracing Function Reference Affecting PHP's Behaviour Audio Formats Manipulation Authentication Services Command Line Specific Extensions Compression and Archive Extensions Credit Card Processing Cryptography Extensions Database Extensions Date and Time Related Extensions File System Related Extensions Human Language and Character Encoding Support Image Processing and Generation Mail Related Extensions Mathematical Extensions Non-Text MIME Output Process Control Extensions Other Basic Extensions Other Services Search Engine Extensions Server Specific Extensions Session Extensions Text Processing Variable and Type Related Extensions Web Services Windows Only Extensions XML Manipulation GUI Extensions Keyboard Shortcuts?

English Brazilian Portuguese Chinese Simplified French German Japanese Romanian Russian Spanish Turkish Other. Edit Report a Bug. Function arguments Information may be passed to functions via the argument list, which is a comma-delimited list of expressions. Passing arguments by reference By default, function arguments are passed by value so that if the value of the argument within the function is changed, it does not get changed outside of the function.

Making a cup of cappuccino. Making a cup of. Making a cup of espresso. Making a bowl of acidophilus raspberry. This can only be used on class and instance methods.

For example, using boolean as a parameter or return type will require an argument or return value that is an instanceof the class or interface boolean , rather than of type bool: Argument 1 passed to test must be an instance of boolean, boolean given, called in - on line 1 and defined in -: C D Fatal error: Argument 1 passed to f must be an instance of C, instance of E given, called in - on line 14 and defined in -: Argument 1 passed to f must implement interface I, instance of E given, called in - on line 13 and defined in -: Strict typing By default, PHP will coerce values of the wrong type into the expected scalar type if possible.

Caution Enabling strict mode will also affect return type declarations. Argument 1 passed to sum must be of the type integer, float given, called in - on line 9 and defined in -: Argument 1 passed to sum must be of the type integer, float given, called in - on line Variable-length argument lists PHP has support for variable-length argument lists in user-defined functions.

This is the only way it is done in C, arrays are always passed as pointers 3. The caller should have it, at least as syntactic sugar. Sometimes, pass by reference could be at the choice of the caller, NOT the function definitition. A function's argument that is an object, will have its properties modified by the function although you don't need to pass it by reference.

You can use very limited signatures for your functions, specifing type of arguments allowed. Also note, that unfortunately "array" is the only built-in type you can use in signature.

Any other types i. An operand and two constants requires evaluation, which is not done by the parser. However, this feature is included as of PHP 5. See this page for more information: This doesn't work in PHP 5 anymore. In the following code I tried to amend this by using the array language-construct as the actual argument in the call to the function.

It is also possible to force a parameter type using this syntax. I couldn't see it in the documentation. It is documented here: In function calls, PHP clearly distinguishes between missing arguments and present but empty arguments. Suppose you want to call the function f many times from function g, allowing the caller of g to specify if f should be called with a specific value or with its default value: The best approach, it seems to me, is to always use a sentinel like null as the default value of an optional argument.

This way, callers like g and g's clients have many options, and furthermore, callers always know how to omit arguments so they can omit one in the middle of the parameter list.

Building upon this, it's also easy to provide fallback behaviors when the argument given is not valid: This might be documented somewhere OR obvious to most, but when passing an argument by reference as of PHP 5. Nothing was written here about argument types as part of the function definition. When working with classes, the class name can be used as argument type.

At least in php 5 -- did not check 4. Argument 1 passed to test:: I like to pass an associative array as an argument. I wondered if variable length argument lists and references works together, and what the syntax might be.

It is not mentioned explicitly yet in the php manual as far as I can find. Too few arguments to function foo , 0 passed in php shell code on line 1 and exactly 1 expected in php shell code: This may be helpful when you need to call an arbitrary function known only at runtime: You can call a function as a variable name.

Hey, I started to learn for the Zend Certificate exam a few days ago and I got stuck with one unanswered-well question.

php - Setting a default value if a variable is empty - Code Review Stack Exchange

This is the question: Actually the use of class or global constants does buy us something. It helps enforce the DRY don't repeat yourself principle. Of course you can fake a global variable for a default argument by something like this: Call-time pass-by-ref arguments are deprecated and may not be supported later, so doing this: The way I ended up using for optional pass-by-ref args is to just pass an unused variable when you don't want to use the resulting parameter value: Follow up to resource passing: It appears that if you have defined the resource in the same file as the function that uses it, you can get away with the global trick.

Here's the failure case: In addition to jcaplan bogus. As of PHP 5. If you prefer to use named arguments to your functions so you don't have to worry about the order of variable argument lists , you can do so PERL style with anonymous arrays: I ran into the problem that jcaplan mentionned. I had just finished building 2 handler classes and one interface. During my testing I realized that my handlers were not initializing their variables to their default values when my interface was calling them with 'null' values: To force your function parameters to take a default value when a null is passed you need to include a conditionnal assignment inside the function definition.

The default value in the declaration becomes more important at this point: Be careful when passing arguments by reference, it can cause unexpected side-effects if one is not careful. I had a program designed to sweep through directories and subdirectories and report on the total number of files, and the total size of all files.

Since it needed to return two values, I used variables passed by reference. This caused a curious bug which took me a while to track down, because the effects of the bug were in a different part of the program than the place where I had made a mistake.

Since the same variable was used for both parameters passed by reference, they ended up both pointing to the same physical location in memory, so changing one of them caused both of them to change. The code below is an excerpt of my program, stripped down to just the few lines necessary to illustrate what was happening: Just like in the usort method.

There is no way how to deal with calling by reference when variable lengths argument list are passed.

Concerning default values for arguments passed by reference: I often use that trick: Note that constants can also be used as default argument values so the following code: Passing constants as default values Works! I tried this in both PHP 4 and 5. It is so easy to create a constant that the php novice might do so accidently while attempting to call a function with no arguments.

I don't notice my typo, the SessionCheck function doesn't work, and it takes me all afternoon to figure out why not! Given that we have two coding styles: And thinking about such use, I might have to think about copying all variables instead of working directly on them Coding that respects function prototypes strictly would, I believe, result in code that is more intuitive to read.

Say you're using type-hinting for an argument, but you want to allow it to be NULL, and you want additional required arguments to the right of it. PHP allows this, as long as you give it the type-hinted argument a default value of NULL.

I have some functions that I'd like to be able to pass arguments two ways: Either as an argument list of variable length e. Only the latter can be constructed on the fly e. The way to do it is to begin the function as follows: If you define your functions in the following way, you can call them whilst only specifying the default parameters you need 1.

Define your function with its mandatory parameters, and an optional array 2. Declare your optional parameters as local variables 3. This line is identical for every function 4.

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The dog stood on the donkey The cat sat on the donkey A dog stood on the mat. DB init done by one, an other changed the DB, and thereafter all others need to use the other DB without creating new instances, or creating a log array in one, and we would like to append the new debug strings to the array, atmany places. How to pass a class as an argument? I needed a way to decide between two possible values for one function parameter.

I didn't want to decide it before calling the function but wanted to do the constraint inside the function call. The parameter must be an instanceof the given class or interface name. The parameter must be an instanceof the same class as the one the method is defined on. The parameter must be an array.

The parameter must be a valid callable. The parameter must be a boolean value. The parameter must be a float ing point number. The parameter must be an integer. The parameter must be a string. The parameter must be either an array or an instanceof Traversable.