[openssl] How to generate an openSSL key using a passphrase from the command line?

First - what happens if I don't give a passphrase? Is some sort of pseudo random phrase used? I'm just looking for something "good enough" to keep casual hackers at bay.

Second - how do I generate a key pair from the command line, supplying the passphrase on the command line?

I finally got it working using these commands, using exec() which it is generally reckoned not safe to use, being better to give the PassPhrase in a file. I can accept this risk as I am sure that the PHP will only ever be executed on my PC (which runs windows & doesn't have a PS command).

openssl genrsa -aes128 -passout pass:foobar -out privkey.pem 2048
openssl rsa -in privkey.pem -passin pass:foobar -pubout -out privkey.pub

Many many thanks to @caf, without whom this would not have been possible.

Only one regret - that, no matter how much I Google, no one can seem to get openssl_pkey_new() working with Xampp on Windows (which is the proper way to generate a key pair)

genrsa has been replaced by genpkey & when run manually in a terminal it will prompt for a password:

openssl genpkey -aes-256-cbc -algorithm RSA -out /etc/ssl/private/key.pem -pkeyopt rsa_keygen_bits:4096

However when run from a script the command will not ask for a password so to avoid the password being viewable as a process use a function in a shell script:

get_passwd() {
    local passwd=
    echo -ne "Enter passwd for private key: ? "; read -s passwd
    openssl genpkey -aes-256-cbc -pass pass:$passwd -algorithm RSA -out $PRIV_KEY -pkeyopt rsa_keygen_bits:$PRIV_KEYSIZE
}