funnyzsh

#!/bin/bash

for shell in zsh ksh mksh dash bash /bin/bash 'busybox sh'; do
echo "$shell":
$shell <<'eof'
set -x
PATH=
echo "PATH=$PATH"
ls
eof
$shell <<'eof'
set -x
PATH=
path=/bin
echo "PATH=$PATH"
ls
eof
echo
done

output

zsh:
+zsh:2> PATH='' 
+zsh:3> echo 'PATH='
PATH=
+zsh:4> ls
zsh: command not found: ls
+zsh:2> PATH='' 
+zsh:3> path=/bin 
+zsh:4> echo 'PATH=/bin'
PATH=/bin
+zsh:5> ls
funnyzsh
output
test
xyz

ksh:
+ PATH=''
+ echo PATH=
PATH=
+ ls
ksh[4]: ls: not found [No such file or directory]
+ PATH=''
+ path=/bin
+ echo PATH=
PATH=
+ ls
ksh[5]: ls: not found [No such file or directory]

mksh:
+ PATH=
+ echo PATH=
PATH=
+ ls
mksh: <stdin>[4]: ls: not found
+ PATH=
+ path=/bin
+ echo PATH=
PATH=
+ ls
mksh: <stdin>[5]: ls: not found

dash:
+ PATH=
+ echo PATH=
PATH=
+ ls
dash: 4: ls: not found
+ PATH=
+ path=/bin
+ echo PATH=
PATH=
+ ls
dash: 5: ls: not found

bash:
+ PATH=
+ echo PATH=
PATH=
+ ls
bash: line 4: ls: No such file or directory
+ PATH=
+ path=/bin
+ echo PATH=
PATH=
+ ls
bash: line 5: ls: No such file or directory

/bin/bash:
+ PATH=
+ echo PATH=
PATH=
+ ls
/bin/bash: line 4: ls: No such file or directory
+ PATH=
+ path=/bin
+ echo PATH=
PATH=
+ ls
/bin/bash: line 5: ls: No such file or directory

busybox sh:
+ PATH=
+ echo PATH=
PATH=
+ ls
funnyzsh
output
test
xyz
+ PATH=
+ path=/bin
+ echo PATH=
PATH=
+ ls
funnyzsh
output
test
xyz