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# extract

(PHP 3>= 3.0.7, PHP 4 , PHP 5)

extract -- Import variables into the current symbol table from an array

## Description

int extract ( array var_array [, int extract_type [, string prefix]])

This function is used to import variables from an array into the current symbol table. It takes an associative array var_array and treats keys as variable names and values as variable values. For each key/value pair it will create a variable in the current symbol table, subject to extract_type and prefix parameters.

> 備注: Beginning with version 4.0.5, this function returns the number of variables extracted.

> 備注: `EXTR_IF_EXISTS` and `EXTR_PREFIX_IF_EXISTS` were introduced in version 4.2.0.

> 備注: `EXTR_REFS` was introduced in version 4.3.0.

extract() checks each key to see whether it has a valid variable name. It also checks for collisions with existing variables in the symbol table. The way invalid/numeric keys and collisions are treated is determined by the extract_type. It can be one of the following values:

`EXTR_OVERWRITE`

  If there is a collision, overwrite the existing variable.

`EXTR_SKIP`

  If there is a collision, don't overwrite the existing variable.

`EXTR_PREFIX_SAME`

  If there is a collision, prefix the variable name with prefix.

`EXTR_PREFIX_ALL`

  Prefix all variable names with prefix. Beginning with PHP 4.0.5, this includes numeric variables as well.

`EXTR_PREFIX_INVALID`

  Only prefix invalid/numeric variable names with prefix. This flag was added in PHP 4.0.5.

`EXTR_IF_EXISTS`

  Only overwrite the variable if it already exists in the current symbol table, otherwise do nothing. This is useful for defining a list of valid variables and then extracting only those variables you have defined out of $_REQUEST, for example. This flag was added in PHP 4.2.0.

`EXTR_PREFIX_IF_EXISTS`

  Only create prefixed variable names if the non-prefixed version of the same variable exists in the current symbol table. This flag was added in PHP 4.2.0.

`EXTR_REFS`

  Extracts variables as references. This effectively means that the values of the imported variables are still referencing the values of the var_array parameter. You can use this flag on its own or combine it with any other flag by OR'ing the extract_type. This flag was added in PHP 4.3.0.

If extract_type is not specified, it is assumed to be `EXTR_OVERWRITE`.

Note that prefix is only required if extract_type is `EXTR_PREFIX_SAME`, `EXTR_PREFIX_ALL`, `EXTR_PREFIX_INVALID` or `EXTR_PREFIX_IF_EXISTS`. If the prefixed result is not a valid variable name, it is not imported into the symbol table.

extract() returns the number of variables successfully imported into the symbol table.

切記!
Do not use extract() on untrusted data, like user-input ($_GET, ...). If you do, for example, if you want to run old code that relies on register_globals temporarily, make sure you use one of the non-overwriting extract_type values such as `EXTR_SKIP` and be aware that you should extract $_SERVER, $_SESSION, $_COOKIE, $_POST and $_GET in that order.

A possible use for extract() is to import into the symbol table variables contained in an associative array returned by wddx_deserialize().

示範 1.extract() example
`<?php /* Suppose that $var_array is an array returned from wddx_deserialize */ $size = "large"; $var_array = array("color" => "blue", "size" => "medium", "shape" => "sphere"); extract($var_array, EXTR_PREFIX_SAME, "wddx"); echo "$color, $size, $shape, $wddx_size\n"; ?>`
The above example will produce:
    blue, large, sphere, medium

The $size wasn't overwritten, because we specified `EXTR_PREFIX_SAME`, which resulted in $wddx_size being created. If `EXTR_SKIP` was specified, then $wddx_size wouldn't even have been created. `EXTR_OVERWRITE` would have caused $size to have value "medium", and `EXTR_PREFIX_ALL` would result in new variables being named $wddx_color, $wddx_size, and $wddx_shape.

You must use an associative array, a numerically indexed array will not produce results unless you use `EXTR_PREFIX_ALL` or `EXTR_PREFIX_INVALID`.

See also compact().

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