Description:
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In the wordpress code, I found a function called absint().
It is not documented in the php manual.

Test script:
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function add_image_size( $name, $width = 0, $height = 0, $crop = FALSE ) {
	global $_wp_additional_image_sizes;
	$_wp_additional_image_sizes[$name] = array( 'width' => absint( $width ), 'height' => absint( $height ), 'crop' => !!$crop );
}