Function Reference/set site transient
Essentially the same as set_transient() but works network wide when using WP Multisite.
One difference is that the transient name should be 40 characters or less in length.
Also, unlike set_transient(), set_site_transient data is stored as "autoload" in the database. This may affect site performance if lots of transients are being used. See the related Trac bug report.