How to read memory usage in htop

Yes, each php process is using 30% of the total memory.

An yes again, shared memory is why you cannot simply add. See the column "SHR", this is the shared memory value, RES stands for resident. From "man top" :

   n: %MEM  --  Memory usage (RES)
      A task's currently used share of available physical memory.


   o: VIRT  --  Virtual Image (kb)
      The total amount of virtual memory used by the task.  It includes all code, data and shared libraries plus pages that have been swapped out.

      VIRT = SWAP + RES.


   p: SWAP  --  Swapped size (kb)
      The swapped out portion of a task's total virtual memory image.


   q: RES  --  Resident size (kb)
      The non-swapped physical memory a task has used.

      RES = CODE + DATA.


   r: CODE  --  Code size (kb)
      The amount of physical memory devoted to executable code, also known as the 'text resident set' size or TRS.


   s: DATA  --  Data+Stack size (kb)
      The amount of physical memory devoted to other than executable code, also known as the 'data resident set' size or DRS.


   t: SHR  --  Shared Mem size (kb)
      The amount of shared memory used by a task.  It simply reflects memory that could be potentially shared with other processes.

Since you weren't using your memory for anything better, its being put to use as disk cache. If you start services, they will just take memory from the cache. This is a good thing, not a sign that anything is wrong. See linuxatemyram