problem with Lock table

my situation is:

i have a web-page A in witch i do some select/insert/update to my mysql db...for this queries i have implemented acid (commit-rollback) and with the lock of the tables i would eliminate the concurrecies...
in an other page B i have the same procedure of page A

I try, with some sleep (PHP), to check if this works fine beetwen the two page...and there aren't problem...all work!!the second page wait until the first does not end

But if i try with the sleep within the page A, and two or more people connect to it..the lock table doesn't work!!All the people do the same sequence of action all together

where am I wrong?
thanks