<s>
The	O
Zend	B-Language
Engine	I-Language
is	O
a	O
compiler	B-Language
and	O
runtime	O
environment	O
for	O
the	O
PHP	B-Application
scripting	O
language	O
and	O
consists	O
of	O
the	O
Zend	O
Virtual	O
Machine	O
,	O
which	O
is	O
composed	O
of	O
the	O
Zend	O
Compiler	B-Language
and	O
the	O
Zend	O
Executor	O
,	O
that	O
compiles	B-Language
and	O
executes	O
the	O
PHP	B-Application
code	O
.	O
</s>
<s>
The	O
first	O
version	O
of	O
the	O
Zend	B-Language
Engine	I-Language
appeared	O
in	O
1999	O
in	O
PHP	B-Application
version	O
4	O
.	O
</s>
<s>
It	O
was	O
written	O
in	O
C	B-Language
as	O
a	O
highly	O
optimized	O
modular	O
back-end	O
,	O
which	O
for	O
the	O
first	O
time	O
could	O
be	O
used	O
in	O
applications	O
outside	O
of	O
PHP	B-Application
.	O
</s>
<s>
The	O
Zend	B-Language
Engine	I-Language
provides	O
memory	O
and	O
resource	O
management	O
,	O
and	O
other	O
standard	O
services	O
for	O
the	O
PHP	B-Application
language	I-Application
.	O
</s>
<s>
Its	O
performance	O
,	O
reliability	O
and	O
extensibility	B-Architecture
played	O
a	O
significant	O
role	O
in	O
PHP	B-Application
's	O
increasing	O
popularity	O
.	O
</s>
<s>
This	O
was	O
followed	O
by	O
Zend	B-Language
Engine	I-Language
2	I-Language
at	O
the	O
heart	O
of	O
PHP	B-Application
5	O
.	O
</s>
<s>
This	O
was	O
followed	O
by	O
Zend	B-Language
Engine	I-Language
3	I-Language
,	O
originally	O
codenamed	O
phpng	O
,	O
which	O
was	O
developed	O
for	O
PHP	B-Application
7	O
and	O
significantly	O
improves	O
performance	O
.	O
</s>
<s>
The	O
newest	O
version	O
is	O
Zend	B-Language
Engine	I-Language
4	O
,	O
which	O
was	O
developed	O
for	O
PHP	B-Application
8	O
.	O
</s>
<s>
The	O
source	O
code	O
for	O
the	O
Zend	B-Language
Engine	I-Language
has	O
been	O
freely	O
available	O
under	O
the	O
Zend	B-Language
Engine	I-Language
License	O
(	O
although	O
some	O
parts	O
are	O
under	O
the	O
PHP	B-Language
License	I-Language
)	O
since	O
1999	O
,	O
as	O
part	O
of	O
the	O
official	O
releases	O
from	O
php.net,	O
as	O
well	O
as	O
the	O
official	O
git	B-Application
repository	O
or	O
the	O
GitHub	B-Application
mirror	O
.	O
</s>
<s>
Various	O
volunteers	O
contribute	O
to	O
the	O
PHP/Zend	O
Engine	O
codebase	O
.	O
</s>
<s>
Zend	B-Language
Engine	I-Language
is	O
used	O
internally	O
by	O
PHP	B-Application
as	O
a	O
compiler	B-Language
and	O
runtime	O
engine	O
.	O
</s>
<s>
PHP	B-Application
Scripts	I-Application
are	O
loaded	O
into	O
memory	O
and	O
compiled	B-Language
into	O
Zend	O
opcodes	B-Language
.	O
</s>
<s>
These	O
opcodes	B-Language
are	O
executed	O
and	O
the	O
HTML	O
generated	O
is	O
sent	O
to	O
the	O
client	O
.	O
</s>
<s>
Zend	O
takes	O
part	O
1	O
completely	O
and	O
a	O
bit	O
of	O
part	O
2	O
;	O
PHP	B-Application
takes	O
parts	O
2	O
and	O
3	O
.	O
</s>
<s>
Zend	O
itself	O
really	O
forms	O
only	O
the	O
language	O
core	O
,	O
implementing	O
PHP	B-Application
at	O
its	O
very	O
basics	O
with	O
some	O
predefined	O
functions	O
.	O
</s>
