# stream_filter_prepend

(PHP 4 >= 4.3.0, PHP 5, PHP 7, PHP 8)

stream_filter_prepend — Attach a filter to a stream

### 说明

stream_filter_prepend(
resource `$stream`,
string `$filtername`,
int `$read_write` = ?,
mixed `$params` = ?
): resource

Adds `filtername` to the list of filters attached to `stream`.

### 参数

`stream`

  The target stream.

`filtername`

  The filter name.

`read_write`

  By default, stream_filter_prepend() will attach the filter to the `read filter chain` if the file was opened for reading (i.e. File Mode: `r`, and/or `+`). The filter will also be attached to the `write filter chain` if the file was opened for writing (i.e. File Mode: `w`, `a`, and/or `+`). `STREAM_FILTER_READ`, `STREAM_FILTER_WRITE`, and/or `STREAM_FILTER_ALL` can also be passed to the `read_write` parameter to override this behavior. See stream_filter_append() for an example of using this parameter.

`params`

  This filter will be added with the specified `params` to the beginning of the list and will therefore be called first during stream operations. To add a filter to the end of the list, use stream_filter_append().

### 返回值

Returns a resource on success or `false` on failure. The resource can be used to refer to this filter instance during a call to stream_filter_remove().

`false` is returned if `stream` is not a resource or if `filtername` cannot be located.

### 注释

> 注意: When using custom (user) filters
> stream_filter_register() must be called first in order to register the desired user filter to `filtername`.

> 注意: Stream data is read from resources (both local and remote) in chunks, with any unconsumed data kept in internal buffers. When a new filter is prepended to a stream, data in the internal buffers, which has already been processed through other filters will not be reprocessed through the new filter at that time. This differs from the behavior of stream_filter_append().

> 注意: When a filter is added for read and write, two instances of the filter are created. stream_filter_prepend() must be called twice with `STREAM_FILTER_READ` and `STREAM_FILTER_WRITE` to get both filter resources.

### 参见

- stream_filter_register() - Register a user defined stream filter
- stream_filter_append() - Attach a filter to a stream

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