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set_post_thumbnail_size( int $width, int $height, bool|array $crop = false )

Registers an image size for the post thumbnail.

$width

(int) (Required) Image width in pixels.

$height

(int) (Required) Image height in pixels.

$crop

(bool|array) (Optional) Whether to crop images to specified width and height or resize. An array can specify positioning of the crop area.

Default value: false

To register additional image sizes for Featured Images use: add_image_size().
To enable featured images, the current theme must include add_theme_support( 'post-thumbnails' ); in its functions.php file. See also Post Thumbnails.
This function will not resize your existing featured images. To regenerate existing images in the new size, use the Regenerate Thumbnails plugin.

function set_post_thumbnail_size( $width = 0, $height = 0, $crop = false ) {
	add_image_size( 'post-thumbnail', $width, $height, $crop );
}

Uses	Description
wp-includes/media.php: add_image_size()	Register a new image size.

Version	Description
2.9.0	Introduced.