<s>
A	O
PHP	B-Language
accelerator	I-Language
is	O
a	O
PHP	B-Application
extension	B-Application
designed	O
to	O
improve	O
the	O
performance	O
of	O
software	O
applications	O
written	O
in	O
the	O
PHP	B-Application
programming	I-Application
language	I-Application
.	O
</s>
<s>
Most	O
PHP	B-Language
accelerators	I-Language
work	O
by	O
caching	B-General_Concept
the	O
compiled	O
opcode/bytecode	O
of	O
PHP	B-Application
representation	O
of	O
php	B-Application
files	O
to	O
avoid	O
the	O
overhead	O
of	O
parsing	O
and	O
compiling	O
source	O
code	O
on	O
each	O
request	O
(	O
some	O
or	O
even	O
most	O
of	O
which	O
may	O
never	O
be	O
executed	O
)	O
.	O
</s>
<s>
To	O
further	O
improve	O
performance	O
,	O
the	O
cached	O
code	O
is	O
stored	O
in	O
shared	B-Operating_System
memory	I-Operating_System
and	O
directly	O
executed	O
from	O
there	O
,	O
minimizing	O
the	O
amount	O
of	O
slow	O
disk	O
reads	O
and	O
memory	O
copying	O
at	O
runtime	O
.	O
</s>
<s>
PHP	B-Language
accelerators	I-Language
substantially	O
increase	O
the	O
speed	O
of	O
PHP	B-Application
applications	O
.	O
</s>
<s>
The	O
effect	O
on	O
application	O
performance	O
of	O
opcode	B-Language
caching	B-General_Concept
varies	O
widely	O
,	O
depending	O
on	O
factors	O
such	O
as	O
the	O
inherent	O
execution	O
time	O
of	O
the	O
PHP	B-Application
application	O
and	O
the	O
percentage	O
of	O
source	O
code	O
actually	O
executed	O
on	O
a	O
given	O
request	O
,	O
and	O
whether	O
additional	O
optimization	O
steps	O
are	O
performed	O
.	O
</s>
<s>
While	O
a	O
code	O
optimizer	O
may	O
even	O
slow	O
down	O
overall	O
performance	O
when	O
used	O
in	O
isolation	O
,	O
it	O
can	O
provide	O
an	O
additional	O
performance	O
boost	O
when	O
coupled	O
with	O
a	O
bytecode	O
cache	B-General_Concept
,	O
as	O
the	O
optimization	O
effort	O
is	O
performed	O
just	O
once	O
.	O
</s>
