Note
The list is not complete, please let me know about errors and omissions.

$_COOKIE	returns array of parameters passed to current web-page via cookies See note on Cookies
$_GET	returns array of parameters from after the ? in current web-page address
$_POST	returns array of parameters passed to current web-page using the <form> option of post
$_SESSION	returns array of parameters held in the SESSION array Consider data security if information is sensitive
/* ... */	comment
//	comment
abs	Absolute value
acos	arc-cosine
acosh	arc-hyperbolic-cosine
array_diff( .., ...)	return values present in either array but not found in the other array
array_intersect( .., ...)	return values present in both arrays
array_key_exists( ..., .. )	returns true/false dependingon whether array ... contains the key ..
array_merge( .. , ... )	merge two (or more) arrays
array_reverse( .. )	Reverse the order of elements in array ..
array_unique( .. )	remove duplicate entries from array ..
array	defines a variable array
arsort( .. )	Sort (descending) the array ..
asin	arc-sine
asinh	arc-hyperbolic-sine
asort( .. )	Sort the array ..
atan	arc-tangent
atan2	arc-tangent-2
atanh	arc-hyperbolic-tangent
base_convert	convert between bases
bindec	binary to decimal
break	Stop a loop (for, while, do, switch etc.)
ceil	round up to nearest integer
checkdate	validate a date
class	defines a class
closedir	close directory
continue	Start next pass of loop (for, while, do, etc.)
cos	cosine
cosh	Hyperbolic cosine
date_parse	Array containing date information
date_sun_info	Array containing sunrise/sunset information
date_sunrise	sunrise
date_sunset	sunset
date	get information about a date
decbin	Decimal to binary
dechex	Decimal to hex
decoct	Decimal to octal
deg2rad	degrees to radians
do{ ... } while( .. );	Repeat ... while .. is true always run ... at least once
easter_date	date of easter
echo	Send to browser as HTML
else	Used as part of an If to indicate action when Not true
elseif	similar to "else { if"
ereg_replace	Replace characters in a string using regular expression rules
ereg	Applies regular expression to string
eregi_replace	Replace characters in a string using regular expression rules (not case sensitive)
eregi	Applies regular expression to string but not case sensitive
exp	exponent of e
expm1	exp(number) - 1
fclose	close a file
fgetcsv	read data from a .csv file
fgets	read data from file
fgets	read one line of data from a file
file_exists( ... )	returns true or false depending on whether the file can be found
file_get_contents	copy the entire contents of a file into a variable
file_put_contents	create a file using the data in a variable
fileatime	returns timestamp when file last accessed
filectime	returns timestamp when file created
filegroup	returns the file group associated with a file
fileinode	returns the files inode
filemtime	returns timestamp when file last modified
fileowner	returns file owner
fileperms	returns file permissions
filepos	get current position for reading/writing within a file
filesize	returns size of file (in bytes)
filetype	returns type of file
floor	round down to nearest integer
fmod	remainder (modulus)
fopen	open a file
for(.. = ..; ..; ..;){ .. }	for ... next loop
foreach(.. as ... => ....){ ..... }	Loop though every element n an array
fputcsv	write data to file in .csv format
fread	read data from file
fseek	set current position for reading/writing within a file
function ..([..[,..]]) { ... }	defines a function with optional parameters
fwrite	write to file
getdate( ... )	break a date down in to month, year, weekday etc. See below for parameters
getowd	get owner of current directory
gettimeofday	time
hexdec	hex to decimal
hypot	hypotenuse of a right-angle triangle
if	if .. then ...
include ..	insert all code contained in the file ..
is_executable	returns the file attribute indicating iof it is executabe
is_file	returns true if a real file and false if a link
is_finite	is number finite
is_infinite	is number infinite
is_link	returns true if file is a link
is_nan	is not a number
is_readable	returns true if file can be read and false if unreadable
is_writeable	returns true if file can be written to
isset( .. )	returns true/false depending on whether the variable exists particularly useful with $_GET, $_POST etc.
krsort( .. )	Sort (descending) the array .. into key sequence
ksort( .. )	Sort the array .. into key sequence
log	Loge
log10	Log10
log1p	log(1 + number)
max(..,..,..)	return largest number in list
max	maximum value
min(..,..,..)	return smallest number in list
min	miniumum value
mktime(..,..,..,..,..,..)	sets a date variable parameters in order are hour,month,second,month,day,year
mt_rand(..,...)	return random integer between .. and ...
mt_srand(..,...)	return random integer between .. and ...
octdec	Octal to decimal
pi	3.141...
pow	Exponential expression
print_r( .. )	output every element in array ..
printf(..)	format the sting .. and print
rad2deg	radians to degrees
rand(..,...)	return random integer between .. and ...
rand	random number (integer)
readdir	read directory entry
return	Return a value from a function
rewinddir	reset directory pointer
round(..[,...])	round number down to ... decimal places nearest integer if ... ommited
round	round to nearest integer
serialize( .. )	Convert array .. into a specially formatted string
Set-Cookie	sets up a cookie See note on cookies
sin	Sine
sinh	Hyperbolic sine
sprintf(..)	format the sting .. uses same settings as printf shown below
sqrt	Square root
srand	seed random number generator
strlen( .. )	Number of characters in string ..
strpos( .., ... )	position of first ... in string .. strings start at character 0
strstr( .., ... )	returns all characters in string .. after(and including) the first ...
strtotime(..)	Convert dates as text into a date variable expects US date formats, unpredictable with UK formats
substr(..,...[,....])	.... letters from string .. staring at position ... strings start at character 0 if .... omitted, all characters to end of string returned
switch (..){ case: ...; break; default: .... }	Select action to take based on ..
tan	Tangent
tanh	Hyperbolic tangent
unserialize( .. )	Convert a serialised string into an array
while( .. ){ ... }	Repeat ... while .. is true

Maths Operators
%	modulus (remainder)
*	multiply
*=	..%=... equivilent to .. = .. % ...
*=	..-=... equivilent to .. = .. * ...
/	divide
/=	..+=... equivilent to .. = .. / ...
++	..++ equivilent to .. = .. + 1 ++.. adds 1 before showing result
+	add
+=	..+=... equivilent to .. = .. + ...
--	..-- equivilent to .. = .. - 1 --.. subtracts 1 before showing result
-	subtract
-=	..-=... equivilent to .. = .. - ...

String Operators
!=	is not the same as
>	is alphabetically after
>=	is alphabetically the same or after
<	is alphabetically before
<=	is alphabetically the same or before
.	concatenate
==	is the same as

File access
a+	read and write file (add to end of file if file already exists)
a	write to file (add to end of file if file already exists)
r+	read and write (starting at beginning of file)
r	read (starting at beginning of file)
w+	read and write file (overwrite entire file if it already exists)
w	write file (overwrite entire file if it already exists)

Conditional operators etc.
!	not
!=	is not equal to
&&	and
>	is greater than
>=	is greater than or equal to
<	is less than
<=	is less than or equal to
||	or
==	is equal to
===	is equal to and the same type of variable
and	and
or	or
xor or	exclusive or

Date(..[,date_variable]) Current date assumed if date_variable omitted Some systems fail on dates before 1970 or after 2037
a	morning/afternoon am-pm
A	morning/afternoon AM-PM
B	internet time
c	full date in ISO format
d	day of month 01-31
D	day of week Sun-Sat
e	time zone identifier
F	name of month January-December
G	hour 0-23
g	hour 1-12
H	hour 00-23
h	hour 01-12
I	Is this daylight saving time
i	minute 00-59
j	day of month 1-31
l	day of week Sunday-Saturday
L	is it a leap year?
M	month Jan-Dec
m	month number 01-12
n	month number 1-12
N	numeric day of week
o	4 digit year adjusted if weeknumber in different year
O	offset from Greenwich Meen Time {hours}
P	offset from Greenwich Mean Time {returns hh:mm}
r	full date in RFC format
S	day on month suffix st-nd-rd-th
s	seconds 00-59
t	number of days in month
T	timezone abbreviation
u	microseconds {needs DateTime::format()}
U	seconds since 1-Jan-1980 (Unix epoch date)
w	number for day of week 0-7 (0-Sunday, 6=Saturday)
W	week number 0-53
y	last 2 digits of year 70-37
Y	year 1970-2037
z	day of year 0-365
Z	timezone offset in seconds

getdate( .. )
hours	hours 0-23
mday	day of month 0-31
minutes	minutes 0-59
mon	month number 1-12
month	January-December
seconds	seconds0-59
wday	number for day of week 0-6 (0=Sun, 6=Sat)
weekday	Sunday-Saturday
yday	day of year 0-365
year	year 1970-2037

printf( .. )
%..	next format must pad to .. characters
b	binary
c	ascii character
d	decimal
e	exponential
f	floating point
o	octal
s	string
u	unsigned
x	hexadecimal (lowercase a-f)
X	hexadecimal (uppercase A-F)

Regular Expressions
.. - ...	any character between .. and ... in computer alphabet
.	any character
?	the previous test is optional
[:alnum:]	a-z, A-Z or 0-9
[:alpha:]	a-z or A-Z
[:digit:]	0-9
[:lower:]	a-z
[:print:]	all normal (printable) characters including space
[:punct:]	all characters except a-z, A-Z, 0-9 and space
[:space;]	space
[:upper:]	A-Z

Cookies
Cookies should be avoided whenever possible for many reasons including these...

• Cookies have been misused in the past and web users are gradually becoming aware of this
• Cookies are not secure, hence totally inappropriate for log-in, shopping carts etc.
• If the Cookie data is that important, store it securely - don't leave it on show for every user of that PC
• Cookies can alienate customers (whilst their use MAY be reasonable, some customers will doubt this)
• Cookies can be blocked by web users. They are more likely to change web-sites than to alter cookie settings
• When browsers detect 3rd Party cookies, customers may get the impression that you have a total disrespect for their personal data
• If you must use cookies, please tell you readers PRECISELY what information you are keeping on their computer and indicate why you need each piece of information