I think the technical answer is no. Rather it is a series of research monographs.
Clearly there is a continuum here, and actually the SLN[X] series seems to have become more journal-like since the last time I checked. (The SLNM webpage lists an inmpact factor, for instance.) I think you can do no better than to consult the series homepages:
Springer Lecture Notes in Computer Science.
Springer Lecture Notes in Mathematics.
Springer Lecture Notes in Physics.
In CS the series seems to be organized into many subseries, however each volume gets a "global number".
However in all three cases you can check the language used and see that they talk about "monographs", "titles" and "texts", never "journals". If my memory is accurate, librarians view them this way as well: sometimes journals can be checked out for a much shorter time than books, and in my experience the SLNM have always been treated like books.
For the math series, from the linked page you can click to get a four page pdf file detailing the editorial policy for the productions of the LNM monographs. The following passage seems rather enlightening:
Monograph manuscripts should be reasonably self-contained and rounded off. Thus they
may, and often will, present not only results of the author but also related work by other
people. They may be based on specialised lecture courses. Furthermore, the manuscripts
should provide sufficient motivation, examples and applications. This clearly
distinguishes Lecture Notes from journal articles or technical reports which normally are
very concise. Articles intended for a journal but too long to be accepted by most journals,
usually do not have this "lecture notes" character. For similar reasons it is unusual for
doctoral theses to be accepted for the Lecture Notes series, though habilitation theses may
I could not find the analogous file for either the SLNCS or SLNP.
In terms of the specific question:
Some universities require the PhD students to publish in journals in order to graduate, so I want to know. I am in Computer Science field.
This is a question about academic culture, both general CS culture and the culture of your specific department and university. You certainly need to ask people in your own local culture. As you can see above, in mathematics graduate students rarely publish in SLNM: PhD theses are generally not appropriate, and it is hard to see what other book-length partially expository high level research document it would be worth the time of a graduate student to write and publish. But it looks like CS does things a bit differently...